# Beyond MACS: Physical properties of extremely X-ray luminous clusters at $z > 0.5$

H Ebeling, J Richard, B Beauchesne, Q Basto, A.C Edge, I Smail

► To cite this version:

H Ebeling, J Richard, B Beauchesne, Q Basto, A.C Edge, et al. Beyond MACS: Physical properties of extremely X-ray luminous clusters at $z > 0.5$. Monthly Notices of the Royal Astronomical Society, 2025, 537 (3), pp.2662-2694. 10.1093/mnras/staf063. hal-04576363

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# Beyond MACS: physical properties of extremely X-ray luminous clusters at $z > 0.5$

H. Ebeling<sup>1</sup>, J. Richard<sup>2</sup>, B. Beauchesne<sup>3</sup>, Q. Basto,<sup>2</sup> A. C. Edge<sup>4★</sup> and I. Smail<sup>4</sup>

Accepted 2024 December 27. Received 2024 December 14; in original form 2024 April 18

## ABSTRACT

We present a sample of over 100 highly X-ray luminous galaxy clusters ($L_{X,0.1-2.4\,\text{keV}} \sim 10^{45} \, \text{erg s}^{-1}$) at $z \sim 0.5-0.9$, discovered by the extended Massive Cluster Survey (eMACS) in ROSAT All-Sky Survey (RASS) data. Follow-up observations of a subset at higher resolution and greater depth with the Chandra X-ray Observatory are used to map the gaseous intracluster medium, while strong-gravitational-lensing features identified in Hubble Space Telescope imaging allow us to constrain the total mass distribution. We present evidence of the exceptional gravitational-lensing power of these massive systems, search for substructure along the line of sight by mapping the radial velocities of cluster members obtained through extensive ground-based spectroscopy, and identify dramatic cases of galaxy evolution in high-density cluster environments. The available observations of the eMACS sample presented here provide a wealth of insights into the properties of very massive clusters ($M_{200} \sim 10^{15} \, \text{M}_\odot$) at $z > 0.5$, which act as powerful lenses to study galaxies in the very distant Universe. We also provide first assessments of the evolutionary state, galaxy populations, and large-scale environment of eMACS clusters and release to the community the cluster sample and supporting spectroscopic catalogues to further the understanding of the first generation of truly massive clusters to have formed in the Universe.

**Key words:** gravitational lensing: strong – galaxies: clusters: general – X-rays: galaxies: clusters.

## 1 INTRODUCTION

Occupying the top spot in the mass-ranked hierarchy of gravitationally collapsed structures in the Universe, massive galaxy clusters play a unique role in numerous areas of astrophysical and cosmological research: they allow us to study the evolution and interaction of baryonic and non-baryonic matter across a wide range of environments and redshifts (e.g. Vikhlinin, Markevitch & Murray 2001; Markevitch et al. 2002; Harvey et al. 2015; Ebeling, Qi & Richard 2017; van Weeren et al. 2019), act as powerful gravitational telescopes that magnify faint background sources out to the very highest redshifts (e.g. Kneib & Natarajan 2011, and references therein), and provide independent constraints on fundamental cosmological parameters (e.g. Vikhlinin et al. 2009; Allen, Evrard & Mantz 2011; Planck Collaboration XX 2014; Mantz et al. 2015; Bocquet et al. 2019). Importantly, they also provide valuable opportunities to observe and understand the evolution of galaxies in high-density environments, from ram-pressure stripping of infalling galaxies (e.g. Gunn & Gott 1972; Vollmer et al. 2001; Cortese et al. 2007; Ebeling, Stephenson & Edge 2014; Boselli, Fossati & Sun 2022) to the mechanisms driving the growth of central cluster galaxies (e.g. Oemler 1976; Merritt 1985; Dubinski 1998; De Lucia & Blaizot 2007; McDonald et al. 2016).

The ability to quantify changes in the ensemble properties of the population of massive clusters with lookback time is crucial for

many of the aforementioned science applications. However, attempts to constrain the physical properties of massive clusters, primarily their mass and relaxation state, at ever-increasing redshifts face the obvious challenge that the most massive clusters ($M \gtrsim 10^{15} \, \text{M}_\odot$) have not had time to form in significant numbers beyond their typical redshift of formation, commonly thought to be $z \sim 1$. As a result, observational studies of such systems beyond the local Universe have, so far, been limited to extreme clusters at the median redshift of $z \sim 0.45$ probed by the X-ray-based<fnref n="1" /> Massive Cluster Survey (MACS; Ebeling, Edge & Henry 2001; Ebeling et al. 2007, 2010; Mann & Ebeling 2012; Repp & Ebeling 2018). At significantly higher redshifts, studies frequently use the term ‘massive cluster’ more loosely, sampling systems of average mass ($\sim 10^{14} \, \text{M}_\odot$), such as those detected in surveys exploiting the Sunyaev–Zel’dovich (SZ; Sunyaev & Zeldovich 1980) effect (e.g. Bleem et al. 2020; Hilton et al. 2021) or in all-sky infrared surveys (e.g. Gonzalez et al. 2019), or targeting the precursors of today’s massive clusters (in their infancy) at redshifts well beyond unity (e.g. Steidel et al. 1998; Papovich et al. 2010; Gobat et al. 2011; Newman et al. 2014; Wang et al. 2016; Morishita et al. 2023).

The extended Massive Cluster Survey (eMACS; Ebeling et al. 2013), described below, was designed to push X-ray searches for very massive clusters to the highest possible redshifts without lowering

<footnote n="1"><sup>1</sup>Institute for Astronomy, University of Hawaii, 640 N. Aohoku Place, Hilo, HI 96720, USA</footnote>

<footnote n="2"><sup>2</sup>Centre de Recherche Astrophysique de Lyon UMR5574, CNRS, Univ Lyon, Univ Lyon1, Ens de Lyon, F-69230 Saint-Genis-Laval, France</footnote>

<footnote n="3"><sup>3</sup>Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne, Observatoire de Sauverny, CH-1290 Versoix, Switzerland</footnote>

<footnote n="4"><sup>4</sup>Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK</footnote>

<footnote n="1">¹An X-ray luminosity of $10^{45} \, \text{erg s}^{-1}$ (0.1–2.4 keV) corresponds roughly to a cluster mass of $10^{15} \, \text{M}_\odot$ (e.g. Lovisari et al. 2020, and references therein).</footnote>

<footnote>© 2025 The Author(s).</footnote>

<footnote>*E-mail: alastair.edge@durham.ac.uk</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Left: distribution of X-ray luminosity versus redshift for cluster samples compiled from RASS data; eMACS extends the extreme $L_X$ regime probed before by, e.g. the eBCS (Ebeling et al. 1997, 2000) and MACS projects (Ebeling et al. 2001) to yet higher redshifts. Right: mass estimates versus redshift for galaxy clusters from SZ-selected samples (adapted from Bleem et al. 2015, but limiting the SPT 2500 deg$^2$ sample to clusters with spectroscopic redshifts and showing $M_{200}$ as opposed to $M_{500}$ in their figure). eMACS was designed to complement existing surveys by focusing on the high-mass, high-redshift regime. Although total masses are not available for all clusters shown in the left-hand plot, the equivalence of an X-ray luminosity of $10^{45}$ erg s$^{-1}$ (0.1–2.4 keV) and a total mass of $\sim 10^{15}$ M$_\odot$ is well established for galaxy clusters (e.g. Applegate et al. 2014).</figcaption></figure>

the typical aimed-at mass of $M_{200} \sim 10^{15}$ M$_\odot$ set at $z > 0.3$ by MACS (Applegate et al. 2014). We here present the eMACS sample as it stands, including an overview of optical, radio, and X-ray follow-up observations of eMACS clusters performed to date. Based on the available data, we give first assessments of the physical properties (mass, evolutionary state, large-scale environment, etc.) of all eMACS clusters for which sufficient high-quality data, from both ground- and space-based follow-up observations, are in hand. We stress that, while the eMACS project aims to find and characterize very massive galaxy clusters at $z > 0.5$, the total mass of approximately $10^{15}$ M$_\odot$ frequently cited in this paper is not a lower threshold but merely a loose characterization of the targeted mass regime (as indicated in Fig. 1). Similar to any flux- or magnitude-limited survey of sources that probes a population whose intrinsic characteristics (mass, luminosity, temperature, etc.) follow a Schechter function, the eMACS sample too includes some of the most massive clusters in the Universe, but is not dominated by the most extreme systems in the sampled parameter space.

This paper is structured as follows. After this brief introduction, we provide an overview of the eMACS project in Section 2, summarize follow-up observations in Section 3, and describe the methods used to analyse the resulting data in Section 4. Section 5 presents the results obtained for the physical properties of eMACS clusters, followed by our conclusions in Section 6. More technical details about the compilation of the eMACS sample from X-ray and optical survey data can be found in Appendix A, while Appendix B serves as a repository for summaries of key observational findings for each cluster.

We assume the Lambda-cold dark matter concordance cosmology, i.e. $\Omega_m = 0.3$, $\Omega_\Lambda = 0.7$, and $H_0 = 100h \text{ km s}^{-1} \text{ Mpc}^{-1}$, with $h = 0.7$.

## 2 EMACS

The distant clusters presented and discussed here were all discovered (or, in a few cases, rediscovered) in the course of the eMACS. eMACS is an X-ray-selected cluster survey based on ROSAT All-Sky

Survey data (RASS; Voges et al. 1999) within a solid angle of over 20 000 deg$^2$, defined by $|b| > 20^\circ$ and $\delta > -30^\circ$. eMACS extends similar work at lower redshifts (e.g. Ebeling et al. 1997, 2000, 2001, 2007, 2010; De Grandi et al. 1999; Reiprich 2001; Ebeling, Mullis & Tully 2002; Böhringer et al. 2004, 2013) to $z > 0.5$ in an attempt to find the first generation of truly massive clusters out to redshifts approaching unity.

Fig. 1 shows the X-ray luminosity, mass, and redshift regime explored by eMACS in comparison with other cluster surveys. The RASS flux limits (0.1–2.4 keV) of the extended Brightest Cluster Sample (eBCS; Ebeling et al. 2000), MACS (Ebeling et al. 2001), and eMACS surveys (this work) are $3 \times 10^{-12}$, $10^{-12}$, and $0.5 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$, respectively. By design, eMACS finds clusters that are as X-ray luminous as the most extreme systems discovered in the eBCS and MACS surveys, but at yet higher redshift. Solid angle is crucial in this context; while the South Pole Telescope (SPT; Carlstrom et al. 2011), a facility designed for the detection of the SZ effect in clusters, is capable of detecting clusters with constant efficiency at all redshifts, the much smaller solid angle covered by the SPT surveys prevents it from finding exceptionally massive clusters in significant numbers. The commonalities and differences between SZ- and X-ray cluster surveys in mass–redshift space are summarized in Fig. 1 which also shows the cluster samples compiled by the SPT, the Atacama Cosmology Telescope (ACT; Swetz et al. 2011), and the Planck satellite. Although Planck covered the entire sky, the mission’s limited angular resolution results in a selection that is biased against clusters even at modest redshift (e.g. Malte Schäfer & Bartelmann 2007; Bleem et al. 2015), creating a strongly redshift-dependent selection function more similar to that of RASS-based cluster samples at moderate redshifts than those of high-resolution SZ surveys such as those conducted with the SPT and ACT. As illustrated in Fig. 1, eMACS does not compete with, but instead complements, SZ cluster surveys. We note that, while all eMACS clusters were discovered by us in the X-ray band without any prior knowledge of their detection in surveys in other bands, we acknowledge that other surveys may have found some of the same systems in optical, near-infrared, or SZ cluster surveys.
In a nutshell, the eMACS sample was compiled as follows. Candidates were selected from the RASS source catalogue<fnref n="2" /> based on very few, simple X-ray selection criteria (beyond the mentioned cuts in Galactic and equatorial latitude, cuts in the spectral X-ray hardness ratio and in the RASS flux in the 0.1–2.4 keV band were applied; see Appendix A for details). Next, visual inspection of optical images covering a 5 arcmin × 5 arcmin area around each source in the publicly available data obtained by the PanSTARRS (PS) facility (Kaiser et al. 2002), specifically the PS1 $3\pi$ survey (Chambers et al. 2016), was used to identify plausible clusters as either overdensities of faint galaxies of similar colour or fields that appear blank to the depth of the PS $3\pi$ data. We refer the reader to Appendix A for more details. As a final step, spectroscopic follow-up observations of this tentative cluster sample were conducted to limit the sample to clusters at a redshift of $z > 0.5$. Results from an eMACS pilot project were presented in Ebeling et al. (2013), and an assessment of the lensing power of eMACS clusters is provided by Zalesky & Ebeling (2020).

References to the 'current' eMACS sample are intended to remind the reader of the sample's incompleteness, which results from several causes, the most fundamental one being the increasingly poor X-ray photon statistics as we approach the detection limit of the RASS. For X-ray sources above the eMACS flux limit of $0.5 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ (0.1–2.4 keV), the incompleteness of the underlying RASS database is 87 per cent, as illustrated by the $\log N - \log S$ distribution shown in Fig. A3 (right). In addition, eMACS clusters may have been missed by flaws in the optical screening of all cluster candidates identified in the X-ray selection process (see Appendix A for details) and by setbacks in the spectroscopic confirmation of these candidates. Although these effects reduce the overall completeness of the sample, we estimate the completeness for the most massive eMACS clusters to still exceed 80 per cent.

In recognition of the wealth of data and insights collected to date, we here release the current eMACS sample (together with all supporting follow-up data) in spite of the residual incompleteness described above. We also hope that this release will prove helpful to the community in light of the progress made in the compilation of cluster samples from the RASS' successor, the next-generation X-ray all-sky survey eROSITA (e.g. Merloni et al. 2024; Bulbul et al. 2024), by preventing unnecessary investments of valuable observing time into the confirmation and characterization of eROSITA detections that would merely duplicate prior eMACS results.

Table 1 lists all 111 entries in the current eMACS sample and provides an overview of selected follow-up observations performed to date (see Section 3 for details). Fig. 2 shows the location of all clusters on the sky and demonstrates that the incompleteness of the sample presented here is not a strong function of celestial position, either in Galactic or equatorial coordinates.

# 3 OBSERVATIONS

## 3.1 Ground-based imaging

By design, all eMACS clusters are covered in five optical passbands ($g'r'i'z'y'$) in the PS1 $3\pi$ survey (Chambers et al. 2016). Although crucial for the identification of cluster candidates at $z \gtrsim 0.5$ among the many faint X-ray sources in the RASS (see Appendix A for details) and sufficient to allow the selection of typically a dozen potential cluster members for spectroscopic follow-up observations

(see Section 3.3), the PS1 $3\pi$ imaging data are too shallow to serve more ambitious purposes.

Consequently, we therefore obtained short (~300 s) observations of all eMACS clusters (i.e. all systems for which the initial round of follow-up spectroscopy found galaxy concentrations at $z \gtrsim 0.5$) to a limiting magnitude of approximately 23 (AB) through the $g'$, $r'$, and $i'$ filters<fnref n="3" /> with the Gemini Multi Object imaging Spectrograph (GMOS) on the 8m Gemini-North telescope on Maunakea. Extending to over 1 Mpc in radius from the centre of a cluster at $z > 0.5$, the 5.5 arcmin field of view of GMOS offers a comprehensive view of the projected spatial distribution of likely cluster galaxies (as identified from their characteristic red-sequence colours) and allowed the efficient selection of targets for more extensive spectroscopic follow-up observations (see Section 3.3). A more detailed description of the GMOS imaging survey of eMACS clusters, as well as of the associated data reduction techniques, is provided by Zalesky & Ebeling (2020) in the context of the mass-traces-light algorithm ASTROLENS that provided a first assessment of the gravitational-lensing strength of eMACS clusters.

## 3.2 HST imaging

Optical and near-infrared imaging with high angular resolution and low sky background is critical for several key science goals of the eMACS project, most notably the detection of strong-gravitational-lensing features, as well as dust and star formation in brightest cluster galaxies (BCGs), and ram-pressure stripping of ‘jellyfish’ galaxies.

To achieve these objectives, eMACS clusters were observed with the Advanced Camera for Surveys (ACS) and the Wide-Field Camera 3 (WFC3) aboard the Hubble Space Telescope (HST) in four SNAPshot programmes (GO-13671, GO-14098, GO-15132, and GO-15843). In addition, eMACS clusters of special interest were targeted by us in dedicated HST imaging campaigns (GO-15466, GO-15608, GO-15844, and GO-16428) that added either depth or area on the sky to existing SNAPshot data. A number of eMACS clusters were also observed independently by other teams (e.g. Hoag et al. 2015; Bleem et al. 2020; Sharon et al. 2020).

The listed SNAPshot programmes requested imaging observations in four passbands: F606W and F814W (ACS; 1200 s each), as well as F110W and F140W (WFC3; 700 s each), although the random nature of SNAPshot surveys meant that not all clusters were observed in all passbands. Imaging was also performed in the F435W passband of ACS, as well as the WFC3 near-infrared filters F105W, F125W, and F160W, in targeted GO observation awarded to us or other teams. An overview of the available HST data in these eight passbands is provided in Table 1. Except for eMACSJ0200.3, which has no HST imaging, all eMACS clusters were observed in at least one passband.<fnref n="4" />

We note that, since the vast majority of the HST images were obtained in randomly selected SNAPshot observations, the resulting HST imaging data set can be considered unbiased and representative of the eMACS sample as a whole. A mild bias is introduced, however, by our efforts to secure multipassband coverage (or greater observational depth) for clusters showing exceptional strong-lensing features in the SNAPshot observations.

<footnote n="2">²Note that eMACS, unlike MACS, draws not only from the RASS Bright Source Catalogue but also from the Faint Source Catalogue.</footnote>

<footnote n="3">³For a small number of clusters, imaging with GMOS was also obtained in the $z'$ passband.</footnote>

<footnote n="4">⁴Scheduled HST observations of eMACSJ1350.7 were cancelled by the proposers after follow-up X-ray observations of the field failed to detect diffuse X-ray emission, thereby ruling out the presence of a galaxy cluster (see Section 5.1.2).</footnote>
<table><caption>**Table 1.** The eMACS sample at the time of publication; we list the cluster redshift, the number of spectroscopic redshifts measured, the total exposure time of all available CXO observations (ACSI-I only), and the optical and NIR passbands in which imaging data were obtained with HST. We also indicate the radio coverage of the clusters, marking as '5 GHz' all systems observed by us using the JVLA at 5 GHz and identifying as '3 GHz' those covered by the VLASS survey (Lacy et al. 2020) at 3 GHz. Clusters with BCGs that are detected in the radio maps at $\ge 5\sigma$ are labelled in italics. The 12 eMACS clusters released earlier as part of the MACS sample (Ebeling et al. 2007) are listed for completeness' sake ($n_z$ entries are deliberately omitted for these 12 clusters). The uncertainty of the listed cluster redshifts is dominated by substructure along the line of sight and can be taken to be 0.001 for the entire sample. eMACS clusters discovered independently by eROSITA (Zaznobin et al. 2023) are marked by a trailing asterisk.</caption><thead><tr><th>Name</th><th>z</th><th>n<sub>z</sub></th><th>t<sub>CXO</sub></th><th>Radio observed (GHz)</th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th></tr><tr><th></th><th></th><th></th><th>(ks)</th><th></th><th></th><th></th><th></th><th>HST passbands</th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td>eMACSJ0014.9-0056</td><td>0.5330</td><td>18</td><td>30</td><td>5</td><td></td><td>F606W</td><td></td><td>F110W</td><td></td><td>F140W</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0018.5+1626</td><td>0.5473</td><td>-</td><td>67</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F125W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0025.4-1222</td><td>0.5848</td><td>-</td><td>158</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F125W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0026.2+0120</td><td>0.6357</td><td>22</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0027.0+1252</td><td>0.6063</td><td>22</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0030.5+2618</td><td>0.4970</td><td>20</td><td>18</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0031.2+1907</td><td>0.5304</td><td>20</td><td></td><td>3</td><td></td><td>F606W</td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0042.5-1103</td><td>0.5701</td><td>13</td><td>12</td><td>3</td><td></td><td></td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0043.1-1129</td><td>0.7282</td><td>19</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0045.2-0151</td><td>0.5471</td><td>15</td><td>49</td><td>5</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0101.1+1831</td><td>0.6533</td><td>23</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0121.4+2143</td><td>0.6011</td><td>17</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0122.7+1454</td><td>0.5469</td><td>15</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0124.0+0430</td><td>0.5363</td><td>14</td><td></td><td>3</td><td></td><td>F606W</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0127.5-0606</td><td>0.5055</td><td>14</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0135.2+0847</td><td>0.6185</td><td>38</td><td>12</td><td>5</td><td></td><td>F606W</td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0153.4+1722</td><td>0.5464</td><td>21</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0200.3-2454</td><td>0.7126</td><td>25</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0224.5-1615</td><td>0.6223</td><td>15</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0241.0+2557*</td><td>0.5752</td><td>22</td><td></td><td>3</td><td></td><td>F606W</td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0248.2+0237</td><td>0.5561</td><td>19</td><td>45</td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0252.4-2100</td><td>0.7017</td><td>26</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0256.7-1623</td><td>0.8621</td><td>18</td><td>33</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0256.9-1631</td><td>0.8670</td><td>17</td><td>33</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0257.1-2325</td><td>0.5056</td><td>-</td><td>38</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F125W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0324.0+2421</td><td>0.9023</td><td>26</td><td>31</td><td>3</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F125W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0325.4-0359</td><td>0.5721</td><td>12</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0403.6+1544*</td><td>0.5275</td><td>20</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0429.0-1011</td><td>0.5418</td><td>10</td><td></td><td>3</td><td></td><td></td><td></td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0454.1-0300</td><td>0.5376</td><td>-</td><td>14</td><td>5</td><td></td><td></td><td>F814W</td><td>F105W</td><td></td><td>F110W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0502.9-2902</td><td>0.6028</td><td>23</td><td>49</td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0647.7+7015</td><td>0.5922</td><td>-</td><td>39</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F110W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0717.5+3745</td><td>0.5454</td><td>-</td><td>243</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F110W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0742.9+5001</td><td>0.6013</td><td>15</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0744.8+3927</td><td>0.6976</td><td>-</td><td>90</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F110W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0747.0+6937*</td><td>0.5696</td><td>13</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0804.6+5325</td><td>0.5786</td><td>20</td><td>4</td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0834.2+4524</td><td>0.6606</td><td>32</td><td>33</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0840.2+4421</td><td>0.6377</td><td>39</td><td>32</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0841.8-0429</td><td>0.5359</td><td>11</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0850.2-0611</td><td>0.5744</td><td>22</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0910.8+3850</td><td>0.5618</td><td>19</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ0911.2+1746</td><td>0.5051</td><td>-</td><td>42</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F125W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0921.6-0621</td><td>0.6842</td><td>15</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0934.6+0540</td><td>0.5627</td><td>15</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0935.1+0614</td><td>0.7787</td><td>21</td><td>30</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ0943.3-1842</td><td>0.5687</td><td>12</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1025.0-1354</td><td>0.6196</td><td>13</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1027.2+0345</td><td>0.7482</td><td>10</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1030.5+5132</td><td>0.5182</td><td>23</td><td>20</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1050.6+3548</td><td>0.5077</td><td>39</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1057.5+5759</td><td>0.6027</td><td>12</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1131.1+3553</td><td>0.5159</td><td>12</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1136.8+0005</td><td>0.5968</td><td>26</td><td>4</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1144.2-2836</td><td>0.5070</td><td>30</td><td>31</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1148.0+5116</td><td>0.5834</td><td>28</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td></td><td></td><td>F110W</td><td>F140W</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ1149.5+2223</td><td>0.5445</td><td>-</td><td>366</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td></td><td>F110W</td><td>F140W</td><td>F160W</td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>**Table 1** – continued</caption><thead><tr><th>Name</th><th>z</th><th>n<sub>z</sub></th><th>t<sub>CXO</sub></th><th>Radio observed (GHz)</th><th></th><th></th><th></th><th></th><th></th></tr><tr><td></td><td></td><td></td><td>(ks)</td><td></td><td></td><td></td><td>HST passbands</td><td></td><td></td></tr></thead><tbody><tr><td>eMACSJ1157.9-1046</td><td>0.5570</td><td>25</td><td>156</td><td>3</td><td>F435W</td><td>F814W</td><td>F110W</td><td>F140W</td><td></td></tr><tr><td>eMACSJ1209.4+2640</td><td>0.5553</td><td>22</td><td>16</td><td>5</td><td></td><td>F105W</td><td>F110W</td><td>F140W</td><td>F160W</td></tr><tr><td>eMACSJ1212.5-1216</td><td>0.6786</td><td>28</td><td></td><td>5</td><td></td><td>F814W</td><td>F110W</td><td>F140W</td><td></td></tr><tr><td>eMACSJ1222.3+2418</td><td>0.5085</td><td>14</td><td></td><td>5</td><td>F606W</td><td></td><td>F110W</td><td>F140W</td><td></td></tr><tr><td>eMACSJ1248.2+0743</td><td>0.5733</td><td>15</td><td></td><td>3</td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td><td></td></tr><tr><td>eMACSJ1251.3+3131*</td><td>0.5069</td><td>16</td><td></td><td>3</td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td><td></td></tr><tr><td>eMACSJ1257.3+3654</td><td>0.5253</td><td>14</td><td></td><td>3</td><td>F606W</td><td></td><td>F110W</td><td>F140W</td><td></td></tr><tr><td>eMACSJ1341.9-2442</td><td>0.8339</td><td>26</td><td>34</td><td>5</td><td>F606W</td><td>F814W</td><td>F105W</td><td>F110W</td><td>F125W</td></tr><tr><td>eMACSJ1350.7-1055</td><td>0.8247</td><td>12</td><td>33</td><td>5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1353.7+4329</td><td>0.7364</td><td>31</td><td>40</td><td>5</td><td>F435W</td><td>F606W</td><td>F105W</td><td>F125W</td><td>F140W</td></tr><tr><td>eMACSJ1407.0-0015</td><td>0.5520</td><td>14</td><td>4</td><td>5</td><td></td><td>F606W</td><td></td><td></td><td></td></tr><tr><td>eMACSJ1414.7+5446</td><td>0.6121</td><td>26</td><td>33</td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F125W</td></tr><tr><td>eMACSJ1419.1-0624</td><td>0.5432</td><td>22</td><td></td><td>5</td><td></td><td>F814W</td><td>F110W</td><td>F140W</td><td>F140W</td></tr><tr><td>eMACSJ1419.2+5326</td><td>0.6384</td><td>32</td><td></td><td>3</td><td></td><td>F814W</td><td>F110W</td><td>F140W</td><td>F140W</td></tr><tr><td>MACSJ1423.8+2404</td><td>0.5434</td><td>-</td><td>19</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F110W</td><td>F125W</td></tr><tr><td>eMACSJ1430.0+4127</td><td>0.6646</td><td>13</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1437.8+0616</td><td>0.5350</td><td>19</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1443.2+0102</td><td>0.5284</td><td>23</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1508.1+5755</td><td>0.5421</td><td>48</td><td>49</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1518.3+2927</td><td>0.6050</td><td>33</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1523.1+1329</td><td>0.5010</td><td>18</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1527.6+2044</td><td>0.6967</td><td>28</td><td>25</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td>F125W</td></tr><tr><td>eMACSJ1621.2-0231</td><td>0.5014</td><td>9</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1621.4+7232*</td><td>0.5863</td><td>34</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1631.1+1528</td><td>0.5087</td><td>11</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1709.5+4731</td><td>0.5527</td><td>40</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1732.4+1934*</td><td>0.5406</td><td>41</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1756.8+4008</td><td>0.5743</td><td>117</td><td>89</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td>F125W</td></tr><tr><td>eMACSJ1757.5+3045*</td><td>0.6105</td><td>32</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1823.1+7822</td><td>0.6754</td><td>20</td><td>22</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td>F125W</td></tr><tr><td>eMACSJ1831.1+6214</td><td>0.8207</td><td>25</td><td>23</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1831.9+5746</td><td>0.7072</td><td>15</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ1852.0+4900</td><td>0.6035</td><td>32</td><td>20</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2018.3-2242</td><td>0.5302</td><td>20</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2020.1-1432</td><td>0.5653</td><td>22</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2025.5-2313</td><td>0.7098</td><td>19</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2026.7-1920</td><td>0.6219</td><td>40</td><td>32</td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2037.1-2534</td><td>0.5263</td><td>36</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2045.8-2438</td><td>0.5917</td><td>20</td><td></td><td>3</td><td></td><td>F814W</td><td>F110W</td><td>F140W</td><td>F140W</td></tr><tr><td>eMACSJ2100.4+0724</td><td>0.5742</td><td>13</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2114.4+0427</td><td>0.5898</td><td>20</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>MACSJ2129.4-0741</td><td>0.5879</td><td>-</td><td>40</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F105W</td><td>F125W</td></tr><tr><td>eMACSJ2137.2-2232*</td><td>0.6131</td><td>23</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2201.9+0711</td><td>0.5633</td><td>11</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2207.5-0302</td><td>0.5304</td><td>21</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>MACSJ2214.9-1359</td><td>0.5022</td><td>-</td><td>38</td><td>5</td><td></td><td>F814W</td><td>F105W</td><td>F125W</td><td>F160W</td></tr><tr><td>eMACSJ2220.3-1211</td><td>0.5292</td><td>35</td><td>12</td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2229.9-0808</td><td>0.6214</td><td>65</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2236.0+3451</td><td>0.7287</td><td>25</td><td></td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2315.3-2128</td><td>0.5400</td><td>43</td><td>31</td><td>3</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2316.6+1246</td><td>0.5258</td><td>32</td><td></td><td>5</td><td></td><td>F606W</td><td>F814W</td><td>F110W</td><td>F140W</td></tr><tr><td>eMACSJ2320.9+2912</td><td>0.4960</td><td>45</td><td>10</td><td>3</td><td></td><td>F814W</td><td>F110W</td><td>F140W</td><td>F140W</td></tr><tr><td>eMACSJ2327.4-0204</td><td>0.7055</td><td>24</td><td>146</td><td>5</td><td>F435W</td><td>F606W</td><td>F814W</td><td>F125W</td><td>F160W</td></tr><tr><td>eMACSJ2351.6-2818</td><td>0.5679</td><td>18</td><td></td><td>3</td><td></td><td>F606W</td><td>F110W</td><td>F140W</td><td>F140W</td></tr></tbody></table>

### 3.3 Optical and near-infrared spectroscopy

Extensive spectroscopic follow-up observations were conducted to target presumed cluster members, potential strong-lensing features, and likely counterparts of X-ray point sources detected in the eMACS cluster fields in the observations with the Chandra X-ray Observatory (C XO) described in Section 3.4. The individual

multi-object spectroscopy targets were selected from HST, GMOS, PS1/3π, and CXO observations. Whenever possible (i.e. depending on the availability and quality of HST and CXO data), priority was given to redshifts measurements of likely strong-lensing features and X-ray point sources, with the remainder of the mask being used to target probable cluster members (selected by their colours).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** The distribution of eMACS clusters in the equatorial sky. The 12 clusters also included in the MACS sample and presented previously in Ebeling et al. (2007) are shown as open symbols. Solid lines delineate the 40-deg wide exclusion zone around the Galactic plane. While the statistical completeness of eMACS has not been accurately quantified, the distribution of clusters across the surveyed solid angle is approximately isotropic.</figcaption></figure>

These observations were performed with the DEep Imaging Multi-Object Spectrograph (DEIMOS; Faber et al. 2003), the Low-Resolution Imaging Spectrometer (LRIS; Oke et al. 1995), and the Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE; McLean et al. 2010, 2012) on the Keck-1 and 2 10 m telescopes on Maunakea, Hawaii'. Near-infrared spectroscopy was also performed with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS; Ichikawa et al. 2006) on the Subaru 8.1-m telescope. In the following, we briefly summarize the instrumental setup for the four spectrographs used.

DEIMOS. 1 arcsec slit width; 600 line mm⁻¹ grating; GG455 order-blocking filter; central wavelength 6300 Å; 1.2 Å pixel⁻¹ sampling;

LRIS. 1 arcsec slit width; D680 dichroic; 300/5000 grism (8.8 Å FWHM resolution, blue arm); 600/7500 grating and central wavelength 8100 Å (4.7 Å resolution, red arm);

MOSFIRE. 0.7 arcsec slit width; I, J, K, and/or H band, depending on the expected wavelength of emission-line features; ABBA dither pattern with (typically) 1.5 arcsec offset; 2.8 Å pixel⁻¹ sampling.

MOIRCS. 1 arcsec slit width; $zJ500$ and/or $HK500$ grisms, depending on the expected wavelength of emission-line features; ABBA dither pattern with 1.5 arcsec offset; 7.7 Å pixel⁻¹ sampling.

Exposure times ranged from approximately 1800 s to several hours, depending on atmospheric conditions, airmass, and the faintness of the primary targets on a given MOS mask. We typically observed all targets first in the optical window and then added near-infrared spectroscopy with MOSFIRE or MOIRCS only for stronglensing features for which no identifiable features were detected in prior observations with LRIS or DEIMOS. An overview of all observing runs for the eMACS project is given in Table 2.

Although we aimed at obtaining, for each eMACS cluster, at least 20 spectroscopic redshifts of cluster members as well as spectroscopic confirmation of at least one gravitational-lensing feature, weather patterns on Maunakea, and the vagaries of the telescope-time allocation process introduced seasonal biases. We also deliberately devoted extra resources to the spectroscopic follow up of eMACS clusters of particular interest, e.g. the most distant ones, the most obvious and most spectacular gravitational lenses, or the most extreme mergers. As a result, the spectroscopic coverage of the sample presented here is uneven, as is apparent from the number of redshifts obtained for each cluster listed in Table 1.

## 3.4 X-ray imaging spectroscopy

We searched the public Chandra Search & Retrieve archive (CHASER<fnref n="5" />) for data collected for eMACS clusters with the Advanced CCD Imaging Spectrometer (ACIS) aboard the CXO; for consistency, we retrieved and analysed only observations performed with the instrument's ACIS-I array in VFAINT mode. Of the resulting list of 47 eMACS clusters observed (see Table 1), the majority were targeted by the eMACS team (PI: Ebeling), with most of the remainder having been selected as part of follow-up observations of detections of the SZ effect. Excluding the 12 systems at $z > 0.5$ discovered previously by the MACS project (Ebeling et al. 2007) and discussed extensively in the literature (e.g. LaRoque et al. 2003; Ebeling et al. 2007; Bradač et al. 2008; Zitrin et al. 2011; Limousin et al. 2012; Ogrean et al. 2016; Wilber et al. 2018; Jauzac et al. 2018), we are left with 35 new systems. For each cluster, we reduce and then merge all available observational data sets.

We emphasize that the resulting subset of eMACS clusters with CXO/ACIS-I data is not representative of the full sample but reflects the scientific interests and target selection criteria put forward by the authors of the underlying CXO proposals. As a result, biases in favour of the most X-ray luminous, the most distant, and the most disturbed clusters are introduced; additional biases may be present but remain unknown to us.

We note that we also searched the *XMM-Newton* archives. Since only 17 eMACS clusters were observed with *XMM-Newton* and all but four of these were also observed with CXO, we opt for keeping the X-ray analysis of our sample uniform and focus exclusively on the CXO observations described above.

## 3.5 Radio observations

In total, 66 of the 111 MACS and eMACS clusters presented here have been observed with the National Radio Astronomy Observatory Jansky Very Large Array (JVLA) at 5 GHz (see Table 1). The majority of these observations (45 of 66) were performed with JVLA in B-array at 5 GHz in 2016 as part of a larger campaign to search for activity in the BCG and other galaxies in MACS and eMACS clusters (PI: Edge, 16A-167). All targets in this project were observed in two visits of 300 s each, separated by 60–80 min, with an associated, nearby phase calibrator. These observations reach a consistent noise level of ~30 µJy beam⁻¹ and an angular resolution of ~2.0 arcsec that depends weakly on declination.

The eMACS survey area was covered several times during the VLA Sky Survey (VLASS; Lacy et al. 2020) at 2–4 GHz, with sources from the first epoch being made available and discussed by Gordon et al. (2021); we scrutinized the associated images for detections of the BCG. The average noise level in the VLASS first epoch is 140 µJy beam⁻¹ with a beam size of 2.5 arcsec, i.e. comparable to that of our deeper JVLA 5 GHz data. All JVLA 5 GHz detections brighter than 0.2 mJy have a matching VLASS 3-GHz counterpart at a significance of at least $2\sigma$.

# 4 DATA REDUCTION AND ANALYSIS TECHNIQUES

## 4.1 Optical imaging

All *HST* data were reduced using standard procedures. Specifically, all available *HST* images for a given field were corrected for

<footnote n="5">⁵https://cda.harvard.edu/chaser/</footnote>
<table><caption>**Table 2.** Observing runs for this programme and eMACS targets observed. The letter in parentheses after the observation date indicates the spectrograph used: D = Keck/DEIMOS, L = Keck/LRIS, M = Keck/MOSFIRE, S = Subaru/MOIRCS. We use a shorthand notation for the eMACS cluster names in which eMACSJ0030.5+2618 becomes e0030.5+26.</caption><thead><tr><th>Date/Obs. run</th><th>Targets</th></tr></thead><tbody><tr><td>2010-11-12(L)</td><td>e0840.2+44</td></tr><tr><td>2011-01-03(D)</td><td>e1057.5+57 e1419.2+53</td></tr><tr><td>2012-07-18(D)</td><td>e1414.7+54</td></tr><tr><td>2013-05-05(D)</td><td>e0921.6-06 e1136.8+00 e1148.0+51 e1341.9-24 e1518.3+29 e1527.6+20 e1621.4+72 e1732.4+19</td></tr><tr><td>2013-09-02(D)</td><td>e0256.7-16 e0256.9-16 e1709.5+47 e1757.5+30 e1823.1+78 e2025.5-23 e2037.1-25 e2137.2-22 e2316.6+12 e2327.4-02</td></tr><tr><td>2014-05-02(D)</td><td>e0834.2+45 e0935.1+06 e0943.3-18 e1027.2+03 e1030.5+51 e1131.1+35 e1144.2-28 e1212.5-12 e1350.7-10 e1419.1-06 e1527.6+20 e1621.2-02 e1831.1+62 e1831.9+57 e1852.0+49</td></tr><tr><td>2014-07-25(D)</td><td>e0026.2+01 e0045.2-01 e0101.1+18 e1631.1+15 e2114.4+04 e2229.9-08 e2236.0+34</td></tr><tr><td>2014-11-15(D)</td><td>e0042.5-11 e0043.1-11 e0252.4-21 e0403.6+15 e0804.6+53 e2201.9+07</td></tr><tr><td>2015-05-19(D)</td><td>e1212.5-12 e1353.7+43 e1756.8+40 e1823.1+78</td></tr><tr><td>2015-06-15(D)</td><td>e1852.0+49 e2018.3-22 e2026.7-19</td></tr><tr><td>2015-07-18(D)</td><td>e1437.8+06 e1831.1+62 e1831.9+57 e2045.8-24 e2100.4+07 e2114.4+04 e2220.3-12 e2315.3-21 e2316.6+12</td></tr><tr><td>2015-11-15(D)</td><td>e0840.2+44 e2229.9-08 e2351.6-28</td></tr><tr><td>2015-12-11(D)</td><td>e0027.0+12 e0200.3-24 e0256.7-16 e0256.9-16 e0834.2+45 e1030.5+51</td></tr><tr><td>2016-01-03(D)</td><td>e0121.4+21 e0122.7+14 e0224.5-16 e0241.0+25 e0403.6+15 e0502.9-29 e0747.0+69 e0841.8-04 e0935.1+06 e1025.0-13 e1057.5+57 e1157.9-10</td></tr><tr><td>2016-02-08(D)</td><td>e0325.4-03 e0429.0-10 e0804.6+53 e0841.8-04 e0850.2-06 e1050.6+35 e1353.7+43</td></tr><tr><td>2016-03-10(L)</td><td>e1222.3+24 e1508.1+57</td></tr><tr><td>2016-05-07(D)</td><td>e1050.6+35 e1248.2+07 e1251.3+31 e1341.9-24 e1350.7-10 e1430.0+41 e1621.4+72 e1823.1+78</td></tr><tr><td>2016-07-04(D)</td><td>e0014.9-00 e1341.9-24 e1353.7+43 e1414.7+54 e1419.1-06 e1430.0+41 e1621.4+72 e1631.1+15 e2020.1-14 e2025.5-23 e2045.8-24 e2137.2-22 e2229.9-08</td></tr><tr><td>2016-07-06(L)</td><td>e1437.8+06 e1756.8+40 e2207.5-03</td></tr><tr><td>2016-09-03(D)</td><td>e0027.0+12 e0252.4-21 e2100.4+07</td></tr><tr><td>2016-09-05(L)</td><td>e0030.5+26 e1732.4+19</td></tr><tr><td>2016-09-19(S)</td><td>e1756.8+40 e2229.9-08</td></tr><tr><td>2016-10-28(D)</td><td>e0030.5+26 e0248.2+02 e2018.3-22 e2229.9-08</td></tr><tr><td>2017-03-20(D)</td><td>e0742.9+50 e1136.8+00 e1144.2-28 e1148.0+51</td></tr><tr><td>2017-05-12(S)</td><td>e0840.2+44 e1057.5+57 e1508.1+57 e1756.8+40</td></tr><tr><td>2017-07-22(D)</td><td>e1443.2+01 e1518.3+29 e2020.1-14 e2037.1-25 e2220.3-12 e2351.6-28</td></tr><tr><td>2017-09-11(M)</td><td>e0256.7-16 e1831.1+62 e2316.6+12</td></tr><tr><td>2017-09-15(D)</td><td>e0043.1-11 e0127.5-06 e0153.4+17 e0324.0+24 e1757.5+30 e1823.1+78 e2026.7-19 e2229.9-08 e2236.0+34 e2315.3-21 e2327.4-02</td></tr><tr><td>2018-01-12(D)</td><td>e0124.0+04 e0200.3-24 e0241.0+25 e0256.7-16 e0256.9-16 e0324.0+24 e0742.9+50 e0850.2-06 e0934.6+05 e1157.9-10 e1212.5-12 e1341.9-24</td></tr><tr><td>2018-06-09(M)</td><td>e1341.9-24 e1508.1+57 e1756.8+40 e1831.1+62</td></tr><tr><td>2018-07-15(D)</td><td>e0031.2+19 e0043.1-11 e1257.3+36 e1419.1-06 e1523.1+13 e1527.6+20 e1756.8+40 e1823.1+78 e2018.3-22 e2020.1-14 e2229.9-08</td></tr><tr><td>2018-09-12(D)</td><td>e0135.2+08 e0324.0+24 e1852.0+49 e2229.9-08</td></tr><tr><td>2018-09-18(M)</td><td>e0324.0+24 e1852.0+49 e2229.9-08</td></tr><tr><td>2018-12-04(D)</td><td>e0031.2+19 e0200.3-24 e0324.0+24 e0850.2-06 e0921.6-06 e2207.5-03 e2316.6+12</td></tr><tr><td>2018-12-15(M)</td><td>e0324.0+24 e0834.2+45 e2316.6+12</td></tr><tr><td>2019-04-25(M)</td><td>e1248.2+07 e1437.8+06 e1756.8+40</td></tr><tr><td>2019-08-22(M)</td><td>e0121.4+21 e0256.7-16 e1508.1+57 e1527.6+20 e1756.8+40 e2037.1-25</td></tr><tr><td>2019-11-14(M)</td><td>e0256.9-16 e0502.9-29 e0850.2-06 e0934.6+05</td></tr><tr><td>2019-11-22(D)</td><td>e0943.3-18 e2201.9+07 e2320.9+29</td></tr><tr><td>2020-08-15(D)</td><td>e0252.4-21 e1756.8+40 e2026.7-19 e2315.3-21</td></tr><tr><td>2020-09-09(M)</td><td>e0252.4-21 e1756.8+40 e2026.7-19 e2315.3-21</td></tr><tr><td>2020-12-11(D)</td><td>e0026.2+01 e0101.1+18 e1050.6+35 e1144.2-28 e2320.9+29</td></tr><tr><td>2021-02-12(D)</td><td>e0502.9-29 e0834.2+45</td></tr><tr><td>2021-02-18(M)</td><td>e0502.9-29 e0834.2+45 e1157.9-10</td></tr><tr><td>2021-05-08(D)</td><td>e0910.8+38 e1209.4+26 e1407.0-00 e1430.0+41</td></tr><tr><td>2021-08-05(D)</td><td>e1443.2+01 e1508.1+57 e1756.8+40 e1823.1+78 e1831.1+62 e2026.7-19</td></tr><tr><td>2021-08-15(M)</td><td>e0252.4-21 e1756.8+40 e2026.7-19 e2315.3-21</td></tr><tr><td>2022-01-08(M)</td><td>e0252.4-21 e0502.9-29 e0834.2+45 e1157.9-10</td></tr><tr><td>2022-08-25(D)</td><td>e0135.2+08 e0256.7-16 e0256.9-16 e0324.0+24 e1756.8+40</td></tr><tr><td>2023-07-13(D)</td><td>e0026.2+01 e0031.2+19 e1621.4+72 e1709.5+47 e1732.4+19 e2037.1-25 e2320.9+29</td></tr></tbody></table>
geometric distortions and mapped to a common coordinate frame tied to the Gaia astrometric reference frame, using the TWEAKREG and ASTRODRIZZLE packages (Fruchter et al. 2010). To provide an overview of the clusters, false-colour RGB images<fnref n="6" /> were then created using the same colour table, scaling, and limits for all fields. We used SEXTRACTOR (Bertin & Arnouts 1996) to generate source catalogues and to obtain photometry in the three chosen passbands. Cluster members were tentatively identified as members of the red sequence (e.g. Visvanathan & Sandage 1977; Bower, Lucey & Ellis 1992) based on colour–magnitude diagrams. We refer the reader to Basto et al. (in preparation) for additional details.

## 4.2 Galaxy spectroscopy

Most of the DEIMOS spectra were reduced using a modified version of the Deep Extragalactic Evolutionary Probe 2 (DEEP2) pipeline (Cooper et al. 2012; Newman et al. 2013), with more recently acquired data having been processed with the PYPEIT package (Prochaska et al. 2020). An early version of PYPEIT was also used to reduce the LRIS data, while the MOSFIRE data reduction was mostly performed with the pipeline designed by the MOSFIRE commissioning team and written by Nick Konidaris with extensive checking and feedback from Chuck Steidel and other MOSFIRE team members (Steidel et al. 2014).

Redshifts were measured and corrected to the heliocentric frame using an adaptation of the SPECPRO package (Masters & Capak 2011). We determined cluster velocity dispersions with the ROSTAT software developed by Beers, Flynn & Gebhardt (1990).

## 4.3 X-ray imaging spectroscopy

Our CXO data reduction extensively uses routines from the CIAO software environment (Fruscione et al. 2006) and largely follows established procedures, beginning with the reprocessing and reprojecting of the archival data, before removing time intervals affected by background flares (we first examine the high-energy 9–12 keV band, then the entire ACIS-I passband). We subsequently identify X-ray point sources as detected by the WAVDETECT source-detection algorithm (Freeman et al. 2002), but employ a visual check to ensure the validity of all point sources in the automatically generated source list.

Since our analysis is similar to the process described in detail in the literature (e.g. Mahler et al. 2023; Beauchesne et al. 2024); we here provide only a brief summary. For each observation, we retrieve the associated ACIS-I blank-sky background from the public archive and compute the response of the instruments within a circle of radius 1 Mpc, initially centred on the cluster position in the RASS source catalogue. To model the background emission (both instrumental and sky backgrounds), we use an empirical model consisting of B-spline functions. For each observation, we build this model by first fitting the blank-sky spectrum for each CCD in the 0.1–13 keV band. We then use a linear combination of these models to define a background model per observation as a linear combination of the CCD models. This linear combination is optimized on the blank-sky spectrum of the chosen cluster region by keeping all coefficients frozen for the spectral fit of the cluster and optimizing only one global amplitude per observation. This procedure allows us to estimate the scaling between the science and the blank-sky observations without relying on the observing

time or the particle background. More details about the background estimation<fnref n="7" /> can be found in appendix A of Beauchesne et al. (2024).

To determine fundamental physical parameters of the intracluster medium (ICM) emission recorded with CXO, we proceed as follows. The overall spectral model is the combination of the previously defined empirical background with a physical model of the cluster emission; we adopt a single-temperature plasma, represented by a single APEC model<fnref n="8" /> (Astrophysical Plasma Emission Code, XSAPEC) with solar abundance ratios from Asplund et al. (2009) and photoelectric absorption parametrised by the column density of neutral hydrogen ($n_H$) along the line of sight (XSPHABS). We fit the combined spectrum to the data in the 0.5–7 keV range using the nested-sampling algorithm provided in the MULTINEST package (Feroz et al. 2019) through its PYTHON wrapper (Buchner et al. 2014). During this fit, the density, temperature ($kT$), and metallicity ($Z$) of the ICM are allowed to vary, in addition to the background normalization. Since $n_H$ is poorly constrained for most clusters, we set it to the Galactic value following Willingale et al. (2013) which adjusts the atomic hydrogen measurements from HI4PI Collaboration (2016) by an additional contribution from molecular hydrogen.

We perform the spectral fitting within a circular area that is determined iteratively. We start by fitting the data within a circle of radius 1 Mpc around the RASS position of each eMACS cluster. From the posterior distribution of the ICM temperature and the cluster redshift, we derive the distributions of $r_{1000}$ and $M_{1000}$ using the relation provided in Arnaud, Aghanim & Neumann (2002). We also compute the centroid of the X-ray emission within the initial fitting area of 1 Mpc with the PYTHON package PYPROFFIT (Eckert et al. 2020) and then define a new extraction region around this centroid position, with a radius given by the median of the $r_{1000}$ distribution. We repeat this process until the new $r_{1000}$ median is included in the median-centred interval of 68 per cent of the previous distribution, and the associated emission centroids are similar. We do not quantitatively monitor the convergence of the centre position, but require the previous centre position to be within 6 arcsec of the new one. In most cases, the differences in the location of the centroid between fits are of the order of 1 arcsec or less. The resulting values of $r_{1000}$ range approximately from 0.5 to 1 Mpc for our sample. The results of this analysis are presented in Table 4.

The spatial distribution of the X-ray emission from eMACS clusters observed with CXO to date is, in all figures in this paper, represented by the isointensity contours of the X-ray surface brightness after adaptive smoothing to $3\sigma$ significance with the ASMOOTH algorithm (Ebeling, White & Rangarajan 2006).

## 4.4 Strong gravitational lensing

We use the multiband high-resolution images obtained with HST (all but four eMACS clusters were observed with HST in at least two passbands; see Section 3.2 and Table 1) to identify potential strongly lensed images based on their similarity in colours and morphologies. The corresponding sources were given high priority for the spectroscopic follow-up observations described in Section 3.3. For the strong-lensing analysis described below we consider the subset of 25 eMACS clusters with at least one multiple-image system confirmed by spectroscopic redshifts.

<footnote n="7">⁷The code used for the reduction of the CXO data, background modelling, and ICM fits is publicly available at https://git-cral.univ-lyon1.fr/lenstool/lenstool/-/tree/X-ray-extension/Chandra_data_processing_example?ref_type=heads.</footnote>

<footnote n="8">⁸http://atomdb.org/</footnote>

<footnote n="6">⁶Whenever possible (see Table 1), we use the F606W, F814W, and F110W images to populate the B, G, and R channels, respectively.</footnote>
To provide a rough first indication of the lensing strength of the eMACS clusters we use the *LENSTOOL* software (Jullo et al. 2007) to create lens models by parametrising the mass distribution of each cluster as a combination of double Pseudo Isothermal Elliptical (dPIE) mass profiles at both large (cluster) and small (galaxy) scales. Following similar work on large cluster samples (e.g. Richard et al. 2010, 2021), we optimize the set of parameters of large-scale dPIE components to best reproduce the locations of the confirmed multiple-image systems in each cluster. To account for the mass in small-scale components, we include each cluster member (selected from the red sequence) as a galaxy-scale perturber of the overall mass distribution. We fix these galaxies’ geometric parameters (centre, ellipticity, and position angle) at the values in the *HST* photometric catalogue, while the remaining parameters of these small-scale dPIEs (velocity dispersion, and cut and core radii) are assumed to follow a light-traces–mass relation with their luminosity.

For 106 eMACS clusters existing multiband *HST* imaging enables us to visually survey the core regions of the clusters for potential strongly lensed background sources, identifiable as highly sheared images (arcs and giant arcs) and multiply imaged sources. Many of these systems are cusp arcs, producing three images, the brightest two of which display mirror symmetry (e.g. eMACSJ0121.4 and eMACSJ1222.3 in Fig. 6). However, higher multiplicity and more complex configurations are seen, such as quads (e.g. eMACSJ1027.2 in Fig. 6) or rare hyperbolic umbilical lensing events as in eMACSJ1248.2 (System 2 in Fig. 3; see also Fig. B53). The significance of such exotic lenses is discussed, e.g. by Lagattuta et al. (2023).

A comprehensive description and discussion of the strong-lensing analysis of eMACS clusters with *LENSTOOL* will be provided in a future paper (Basto et al., in preparation).

# 5 RESULTS AND DISCUSSION

In the following, we present our findings on the X-ray point-source contamination, gravitational-lensing strength (as estimated from our strong-lensing analysis), the properties of the ICM, and the galaxy populations of eMACS clusters.

We stress again that the eMACS sample presented here is not statistically complete; in addition, the sample's coverage in follow-up observations (be it galaxy spectroscopy, *HST* imaging, or X-ray imaging spectroscopy with *CXO*) is currently inhomogeneous. Given these limitations, the goal of this paper is not a comprehensive characterization of the population of X-ray luminous clusters at $z > 0.5$. Rather, we aim to highlight subsets of the sample as well as individual systems that were found to be exceptional, warrant further study, or yield insights into the selection biases of cluster samples compiled at different wavelengths and using different survey techniques.

The statistical completeness (or lack thereof) of eMACS and any eMACS subsample is strongly affected by both statistical and systematic uncertainties in the RASS data from which cluster candidates were selected (see Section 2 and Appendix A for details). The most important contributors are shot noise (some RASS detections of eMACS clusters are comprised of a dozen or fewer net X-ray photons) and contamination by X-ray point sources, a systematic issue that we describe and assess in the following section.

## 5.1 CXO versus RASS

Since ROSAT's effective point spread function (PSF) is significantly broadened during the RASS compared to the on-axis

PSF of the system (and cannot be easily modelled, De Grandi et al. 1997), the standard source-detection algorithms used in the RASS reliably recognize only relatively nearby and X-ray bright clusters as intrinsically extended sources (Ebeling et al. 1996). Moreover, the RASS X-ray fluxes and luminosities of clusters even at $z < 0.3$ are prone to contamination from unresolved X-ray point sources, be they embedded in the cluster or physically unrelated fore- or background sources. At the far higher redshifts and dramatically poorer photon statistics probed by eMACS, the impact of X-ray point sources on our RASS-selected sample is a priori unquantifiable but can be measured through follow-up observations with *CXO* that allow point sources to be resolved and removed.

Fig. 4 demonstrates that eMACS delivers on the promise of Fig. 1 by showing a comparison of the cluster luminosities as observed in the RASS and in the dedicated follow-up observations performed with *CXO* of selected eMACS clusters. Here, we have removed all point sources from the *CXO* data prior to measuring the luminosity of the diffuse ICM emission, whereas (by necessity) no such correction was applied to the RASS luminosities. We find that, in general, contamination from X-ray point sources is a minor effect,<fnref n="9" /> as no significant offset from the identity relation (shown as a dotted line in Fig. 4) is seen in the correlation, and the scatter about it is largely consistent with the statistical uncertainties in the RASS-based measurements, with the exception of the four systems labelled in Fig. 4 and discussed below.

### 5.1.1 Flux contamination

We highlight in Fig. 4 two clusters (eMACSJ1144.2 and eMACSJ2315.3) for which the point-source corrected X-ray luminosity as measured from *CXO* observations falls below the RASS estimate by over a factor of five. Even for such outliers, however, the cause of the discrepancy is not always readily apparent. As shown in Figs B47 and B95, several X-ray point sources contribute to the X-ray flux measured for these two clusters at the low angular resolution of the RASS; however, the total flux from all X-ray point sources out to 2 arcmin radius from the peak of the ICM emission amounts to less than 10 per cent of the reported RASS fluxes in Fig. 4. Then again, since many types of X-ray point sources are time variable, any of the point sources apparent in Figs B47 and B95 could have been substantially brighter in the early 1990s when the RASS was conducted.

The degree of point-source contamination in X-ray-selected cluster surveys may also depend on the mass and evolutionary state of the systems under study. Low-mass clusters and groups of galaxies are less dense and dynamically younger; as a result, late-type galaxies tend to be more prevalent and nuclear activity (in cluster members as well as in galaxies in the cluster environment) may be stronger and more common. Anecdotally, the point source contamination appears indeed to be more pronounced in the RASS-selected MACS and BCS cluster samples which, probing lower redshifts, sample a broader range of cluster masses than eMACS (e.g. Ebeling et al. 2010).

### 5.1.2 Misidentifications

While the relative flux contributions from the diffuse ICM and from related or unrelated point sources can be difficult to disentangle for

<footnote n="9">⁹For the sample presented here, the median flux from point sources within the RASS detect cell is less than $5 \times 10^{-15} \text{ erg cm}^{-2} \text{ s}^{-1}$ (0.1–2.4 keV).</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** HST image of the core of eMACSJ1248.2 at $z = 0.573$ (40 arcsec × 30 arcsec, see also Fig. B53). The components of three multiple-image systems are marked, among them a rare hyperbolic-umbilical configuration (System 2, with four images: 2.1–2.4), although unfortunately this currently lacks a spectroscopic redshift.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Comparison of eMACS cluster luminosities in the 0.1–2.4 keV band, as derived from RASS and CXO observations. The most extreme outliers are labelled and discussed in Section 5.1.1. Two CXO non-detections (see Section 5.1.2) are plotted as upper limits at fiducial CXO luminosities of $1.2 \times 10^{44}$ erg s$^{-1}$. The distribution in RASS X-ray luminosity of the eMACS clusters without CXO/ACIS-I data is shown as a histogram. Note that X-ray point sources were removed before measuring cluster luminosities in the high-resolution CXO data, whereas the luminosities observed in the RASS include X-ray point sources (where present).</figcaption></figure>

individual clusters (but does not invalidate the presence and discovery of the respective cluster), a total absence of ICM emission renders the respective entry in our catalogue a plain misidentification.

Based on the current CXO follow-up observations, we are aware of two such cases: as noted in Fig. 4, eMACSJ0256.9 ($z = 0.8670$) and eMACSJ1350.7 ($z = 0.8247$) show no significant extended X-ray emission in 33 ks observations with CXO. We show the HST image

of the former (an already awarded HST observation of the latter was cancelled by us after the CXO observation failed to detect ICM emission) as well as the histogram of radial velocities of galaxies in this region in Fig. B22. Note the absence of ICM emission from eMACSJ0256.9 (Fig. B22) in spite of an obvious overdensity of galaxies at the catalogued RASS source position and the high velocity dispersion of $1170_{-240}^{+140}$ km s$^{-1}$ measured for the structure (based on 17 concordant redshifts). eMACSJ0256.9 thus underlines the critical importance of moderately deep follow-up observations with a high-resolution X-ray facility like CXO to identify non-clusters: as is apparent from Fig. B22, the X-ray flux detected in the RASS is in fact generated by several X-ray point sources, the brightest of which was spectroscopically identified by us as a QSO at $z = 0.8562$, i.e. at the same redshift of the purported cluster. We discuss eMACSJ0256.9 further in Section 5.5.2 in the context of large-scale structure.

## 5.2 Strong lensing

Given the size of the eMACS sample (and the wealth of strong-lensing features present), a comprehensive strong-lensing study of all eMACS clusters not only requires significant observational and analytical resources but also faces systematic challenges. Although compelling signs of strong gravitational lensing are found in the majority of eMACS clusters for which the prerequisite HST imaging is available (Fig. 6), the limitations of the data available for some clusters (e.g. no colour information; no spectroscopic redshifts; clipping of the strong-lensing area by the WFC3 aperture) hampers the secure identification of the most powerful lensing constraints: multiple-image systems. Nevertheless, for clusters with at least one spectroscopically confirmed multiple-image system, we are able to create tentative lens models as described in Section 4.4. A more detailed description of strong-lensing constraints and the resulting LENSTOOL models is provided by Basto et al. (in preparation).

### 5.2.1 Lensed sources

Strong lensing by massive galaxy clusters offers unique insights into the properties of background galaxies in two complementary ways: by probing the background population in a statistical manner, thereby providing valuable constraints on, for instance, the galaxy luminosity function at redshifts that are not observationally accessible in other ways (e.g. Coe et al. 2019; Bouwens et al. 2022), and by enabling detailed studies of the physical properties of individual, intrinsically faint background sources at more modest redshifts (e.g. Alavi et al. 2014). If quantitative insights are aimed for, both of these avenues of research require credible models of the mass distribution within the cluster lenses, and hence the identification of unambiguous strong-lensing features to constrain these lens models (see Section 4.4). While exploiting the former, statistical goal requires much deeper follow-up observations (such as those of the Frontier Fields project. Lotz et al. 2017, or the JWST UNCOVER programme, Bezanson et al. 2024) than have been performed for eMACS to date, first steps toward the latter goal can be taken with the data at hand.

The colours and morphologies of the strongly lensed features showcase the diversity of high-redshift sources seen through the cores of massive clusters at $z > 0.5$ and, in rare cases, can even reveal extended background structures, such as the galaxy group at $z = 2.55$ lensed by eMACSJ1437.8 (Fig. 5 and Fig. B64). For background sources at modest redshifts, typically at $z \le 2$, the larger apparent sizes and in general brighter magnitudes allow complex internal structures of the sources to be readily apparent
<figure><img src="image_6.png" /><figcaption>**Figure 5.** HST image of the core of eMACSJ1437.8 at z = 0.535 (50 arcsec × 50 arcsec, see also Fig. B64), revealing a group of background galaxies at z = 2.55 gravitationally lensed by the foreground cluster. The components of three multiple-image systems (two of them, Systems 1 and 2, spectroscopically confirmed at z = 2.546 and 2.556) are marked.</figcaption></figure>

through the mirror symmetry and image shear that result from
strong gravitational lensing; examples are the spiral galaxy at
z = 1.195 behind eMACSJ1212.5 (Fig. 6 and Fig. B51), or the
quiescent galaxy at z = 1.594 behind eMACSJ1341.9 (discussed
in Ebeling et al. 2018; see also Fig. 6 and Fig. B56), although
resolved galaxy features can be seen out to higher redshifts (e.g.
the fold arc at z = 3.201 behind eMACSJ2229.9, Fig. 6 and Fig.
B93). At higher redshifts, the combination of strong image shear
and increasingly compact sources sizes (particularly in the rest-
frame UV at z ≳ 2), as well as surface-brightness dimming, con-
tribute to the apparent narrowness of some lensed features, such as
the complex ‘straight arc’ at z = 2.03 in eMACSJ1756.8 (Fig. 6
and Fig. B75), or highly sheared examples like the giant arc in
eMACSJ2327.4 at z = 2.983 (Sharon et al. 2015, see also Fig. 6 and
Fig. B98).

5.2.2 Einstein radii and systematic effects

As a first indicator of central lensing strength, we list in Table 3 the Einstein radius, $R_E$,<fnref n="10" /> of the 25 eMACS clusters modelled to date with LENSTOOL, estimated for a fiducial source at $z=2$; in addition, we also list the total projected gravitational mass within that radius. Both quantities exhibit a large spread for reasons that include true diversity in the clusters’ physical characteristics<fnref n="11" /> as well as uncertainties in the model assumptions for cluster lenses currently constrained by only a single multiple-image system. We note, however, that we also make use of multiple-image systems without spectroscopic redshifts

(where available) by iteratively improving the lens model based on
comparisons of the observed image locations with those predicted by
the lens model for an estimated redshift for the respective multiple-
image system (for more details on this established technique, see e.g.
Richard et al. 2010, 2021; Fox et al. 2022).

By focusing on the Einstein radius we probe only the very central
cluster region ($r \lesssim 200$ kpc) where we find enclosed masses from
approximately $2 \times 10^{13}$ to $5 \times 10^{14}$ M$_\odot$. The virial masses ($M_{200}$)
of these systems can be expected to be higher by typically a factor
of ~20 (Wiesner et al. 2012), although this factor strongly depends
on the concentration of the mass profile. eMACS clusters with large
Einstein radii therefore hold the potential to reach virial masses of up
to a few $10^{15}$ M$_\odot$, a prediction that could be tested with the help of
weak-lensing analyses and hybrid mass models that combine strong
and weak lensing (e.g. Niemiec et al. 2020).

We demonstrate that both real physical cluster characteristics and the lens-modelling methodology affect the derived Einstein radii by comparing the Einstein radii from our LENSTOOL analysis to those obtained in a previous indirect strong-lensing analysis of almost the entire eMACS cluster sample with the ASTROLENS package (Zalesky & Ebeling 2020). Unlike LENSTOOL, ASTROLENS uses a light-traces-mass approach that, although calibrated using a small number of clusters with spectroscopically confirmed strong-lensing features, relies entirely on the spatial distribution of light from the cluster galaxies (as measured from ground-based imaging) to derive mass models for eMACS clusters. As such, ASTROLENS provides an estimate of the global lensing power on scales determined by the extent of the distribution of cluster galaxies. LENSTOOL, by contrast, models the mass distribution in much greater detail from actual strong-lensing features—but only within the strong-lensing regime, i.e. the cluster core. Comparing the Einstein radii estimated by the two methods for the eMACS subsample listed in Table 3, we find a systematic offset between the results from the two methods, in the sense that ASTROLENS on average overestimates the Einstein radius by a factor of $1.26 \pm 0.12$. Once this bias is removed,$^{12}$ the results show an overall correlation that is reassuring, given the differences in the methodologies of the two analyses and in the underlying observational data (Fig. 7). The scatter about the identity line (characterized by $R_{E,LTM}/R_{E,PM} = 1.00 \pm 0.28$, where $R_{E,LTM}$ denotes the rescaled Einstein radii obtained with ASTROLENS) is substantial though and indicative of small-scale structure in the majority of eMACS clusters, as expected for a population of clusters that are actively growing through mergers and accretion (see Sections 5.3 and 5.4).

While the Einstein radii determined by the two methods are in
good agreement once the mentioned systematic bias is removed,
Fig. 7 reveals a handful of extreme outliers that warrant discussion.
We refer to Sections 5.4.2 and 5.4.5 for a brief review of the cases of
the obvious mergers eMACSJ0834 and eMACSJ1353 but highlight
the uncertainty created by the degeneracy of a simplistic, single-
constraint lens model for the most extreme outlier, eMACSJ1212.5,
in Fig. 8. The cause of the discrepancy in the $R_E$ values of the
remaining two clusters labelled in Fig. 7, eMACSJ1157.9 (Fig. B49)
and eMACSJ1756.8 (Fig. B75) is of a different nature and already
hinted at by them being flagged as lower limits in Table 3. In both
cases, a large component of the cluster is missing from the LENSTOOL
model due to a lack of (spectroscopically confirmed) strong-lensing

<footnote n="10">¹⁰Following, e.g. Meneghetti et al. (2023), we compute the equivalent Einstein radius $R_E$ as the radius of the circular aperture enclosing the same area as the tangential critical line at this redshift.</footnote>

<footnote n="11">¹¹Including the cluster environment, as strong lensing is sensitive to the total projected mass along the line of sight.</footnote>

<footnote n="12">¹²We note that the subsample of Table 3 includes 9 of 10 eMACS clusters that were used in the calibration of ASTROLENS (the 10th one being eMACSJ0850.2, Fig. B34), and that discrepancies for this subset were, in part, apparent and acknowledged in Zalesky & Ebeling (2020).</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Examples of strong-lensing features in *HST* images of eMACS clusters. We use a shorthand notation for the cluster names.</figcaption></figure>
<table><caption>**Table 3.** Einstein radii at $z_{\text{source}} = 2$ and enclosed masses from our strong-lensing analysis of all eMACS clusters with spectroscopically confirmed multiple-image systems (PM = parametric model). We show entries as lower limits where an obviously present second mass component is not included in our lens model due to a lack of strong-lensing constraints. Also listed is $N_{\text{mi}}$, the number of multiple images used to constrain each model, and $N_{\text{mi,z}}$, the number of such images belonging to spectroscopically confirmed multiple-image systems.</caption><thead><tr><th>Name</th><th><i>R</i><sub>E,PM</sub><br>(arcsec)</th><th><i>M</i>(<i>r</i> &lt; <i>R</i><sub>E,PM</sub><br>(10<sup>12</sup> M<sub>☉</sub>)</th><th><i>N</i><sub>mi</sub></th><th><i>N</i><sub>mi,z</sub></th></tr></thead><tbody><tr><td>eMACSJ0030.5+2618</td><td>9 ± 2</td><td>16</td><td>4</td><td>4</td></tr><tr><td>eMACSJ0121.4+2143</td><td>12 ± 1</td><td>40</td><td>3</td><td>3</td></tr><tr><td>eMACSJ0252.4-2100</td><td>15 ± 2</td><td>72</td><td>12</td><td>12</td></tr><tr><td>eMACSJ0256.7-1623</td><td>11 ± 1</td><td>49</td><td>12</td><td>12</td></tr><tr><td>eMACSJ0324.0+2421</td><td>11 ± 2</td><td>51</td><td>3</td><td>3</td></tr><tr><td>eMACSJ0502.9-2902</td><td>25 ± 3</td><td>164</td><td>20</td><td>9</td></tr><tr><td>eMACSJ0834.2+4524</td><td>32 ± 4</td><td>324</td><td>9</td><td>9</td></tr><tr><td>eMACSJ0840.2+4421</td><td>14 ± 1</td><td>58</td><td>8</td><td>8</td></tr><tr><td>eMACSJ0934.6+0540</td><td>16 ± 2</td><td>60</td><td>10</td><td>7</td></tr><tr><td>eMACSJ0943.3-1842</td><td>&gt; 12</td><td>&gt; 37</td><td>3</td><td>3</td></tr><tr><td>eMACSJ1057.5+5759</td><td>20 ± 10</td><td>119</td><td>4</td><td>2</td></tr><tr><td>eMACSJ1157.9-1046</td><td>&gt; 15</td><td>&gt; 44</td><td>9</td><td>3</td></tr><tr><td>eMACSJ1209.4+2640</td><td>22 ± 2</td><td>131</td><td>19</td><td>7</td></tr><tr><td>eMACSJ1212.5-1216</td><td>40 ± 4</td><td>542</td><td>3</td><td>3</td></tr><tr><td>eMACSJ1248.2+0743</td><td>17 ± 2</td><td>69</td><td>11</td><td>3</td></tr><tr><td>eMACSJ1341.9-2442</td><td>16 ± 2</td><td>79</td><td>5</td><td>5</td></tr><tr><td>eMACSJ1353.7+4329</td><td>38 ± 5</td><td>525</td><td>13</td><td>4</td></tr><tr><td>eMACSJ1437.8+0616</td><td>14 ± 1</td><td>47</td><td>11</td><td>8</td></tr><tr><td>eMACSJ1527.6+2044</td><td>21 ± 2</td><td>136</td><td>7</td><td>2</td></tr><tr><td>eMACSJ1756.8+4008</td><td>&gt; 19</td><td>&gt; 79</td><td>7</td><td>7</td></tr><tr><td>eMACSJ1852.0+4900</td><td>15 ± 2</td><td>54</td><td>5</td><td>5</td></tr><tr><td>eMACSJ2229.9-0808</td><td>&gt; 11</td><td>&gt; 27</td><td>9</td><td>6</td></tr><tr><td>eMACSJ2316.6+1246</td><td>22 ± 3</td><td>114</td><td>9</td><td>9</td></tr><tr><td>eMACSJ2327.4-0204</td><td>35 ± 4</td><td>420</td><td>12</td><td>12</td></tr></tbody></table>

features. Weak-lensing follow-up studies will be able to overcome this limitation and might also benefit other eMACS clusters that appear to contain multiple or additional cluster-scale components that lack suitable strong-lensing features, such as eMACSJ0943.3 (Fig. B39), eMACSJ1831.1 (Fig. B78), or eMACSJ2229.9 (Fig. B93).

While the short list of eMACS clusters with LENSTOOL models presented here contains several exceptional gravitational lenses, many other eMACS clusters exhibit strong-lensing features that make them promising candidates for in-depth follow-up studies. Among the systems with existing HST images, such noteworthy clusters are eMACSJ0014.9 (Fig. B1), eMACSJ0031.2 (Fig. B5), eMACSJ0121.4 (Fig. B10), eMACSJ0747.0 (Fig. B29), eMACSJ0850.2 (Fig. B34), eMACSJ0934.6 (Fig. B37), eMACSJ1027.2 (Fig. B41), eMACSJ1131.1 (Fig. B45), eMACSJ1144.2 (Fig. B47) eMACSJ1222.3 (Fig. B52), eMACSJ1414.7 (Fig. B60), eMACSJ1419.2 (Fig. B62), eMACSJ1508.1 (Fig. B66), eMACSJ1732.4 (Fig. B74), and eMACSJ1823.1 (Fig. B77).

## 5.3 ICM properties and relaxation state

An overview of the key cluster properties derived from the X-ray follow-up observations described in Section 3.4 is given in Table 4. Since most of the underlying CXO observations were obtained with the explicit (and modest) goal of establishing point-source-subtracted cluster luminosities and assessing the large-scale morphology of the ICM, the chosen ACIS-I exposure times are relatively short for such faint targets.

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Comparison of Einstein radii estimated for a fiducial background source at $z=2$ for 25 eMACS cluster using the parametric-modeLing software LENSTOOL ($R_{\text{E,PM}}$) and the light-traces-mass package ASTROLens ($R_{\text{E,LTM}}$). The dotted line marks the identity relation; the five most extreme outliers, labelled in this graph, are discussed in Section 5.2. The vertical error bar in the upper left indicates the approximate statistical uncertainty of 25 per cent of the $R_{\text{E,LTM}}$ values. Note that all ASTROLENS measurements shown in this graph were scaled down by a factor of 1.26 to remove a systematic bias relative to the LENSTOOL results.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 8.** The core of eMACSJ1212.5 (cf. Fig. B51) with dramatically differing tangential critical lines for strong gravitational lensing of a background source at $z = 1.195$ (the redshift of the observed multiple-image system) and $z = 2$ overlaid in cyan and yellow, respectively. For each source redshift, the critical line is shown for cluster core radii of 60, 100, and 200 kpc. By design, the critical lines for $z_{\text{src}} = 1.195$ satisfy the strong-lensing constraints for any core radius.</figcaption></figure>
<table><caption>**Table 4.** Global physical characteristics of eMACS clusters from CXO/ACIS-I data available at the time of publication; $n_H$ values represent the Galactic column density of atomic and molecular hydrogen based on H14PI Collaboration (2016) and Willingale et al. (2013); all X-ray fluxes (unabsorbed) and luminosities are listed in units of $10^{-13}$ erg s$^{-1}$ cm$^{-2}$ and $10^{44}$ erg s$^{-1}$, respectively. Errors correspond to $1\sigma$ confidence. We list the properties of the two components of the double cluster eMACS11823 separately; note, however, that the listed RASS flux (marked with a 「f」symbol) is the same and duplicated for both entries, since this source is not resolved in the RASS. See Section 5.3.2 and Ebeling et al. (2007) for a description of the morphological classification listed in the final column. Morphology class entries followed by a question mark are likely to be overestimated; the four clusters affected were observed in snapshot observations awarded to teams who placed these extended targets at the default aipoint of the ACIS-I array, causing a significant fraction of the ICM flux to be lost in the CCD chip gaps. An estimate of the total cluster mass within $r_{200}$ can be obtained by doubling $M_{1000} = M(<r_{1000})$ (see Section 5.3).</caption><thead><tr><th>Name</th><th>z</th><th>n<sub>H</sub><br>(10<sup>20</sup> cm<sup>-2</sup>)</th><th>L<sub>X-RASS</sub><br>[0.1-2.4 keV]</th><th>t<sub>ACIS-I</sub><br>(ks)</th><th>r<sub>1000</sub><br>(kpc)</th><th>f<sub>X-CXO</sub><br>[0.5-7 keV]</th><th>L<sub>X-CXO</sub><br>[0.5-7 keV]</th><th>kT<sub>X-CXO</sub><br>(keV)</th><th>M<sub>x-CXO</sub><br>(10<sup>44</sup> M<sub>☉</sub>)</th><th>Morph.<br>class</th></tr></thead><tbody><tr><td>eMACS0014.9-0056</td><td>0.5330</td><td>3.37</td><td>6.2 ± 1.9</td><td>29.3</td><td>880<sup>+69</sup><sub>-70</sub></td><td>11.6<sup>+0.5</sup><sub>-0.6</sub></td><td>13.0<sup>+0.6</sup><sub>-0.7</sub></td><td>10.1<sup>+1.6</sup><sub>-1.5</sub></td><td>6.9<sup>+1.8</sup><sub>-1.5</sub></td><td>2</td></tr><tr><td>eMACS0030.5+2618</td><td>0.4970</td><td>3.96</td><td>4.6 ± 1.7</td><td>15.6</td><td>615<sup>+72</sup><sub>-62</sub></td><td>3.7<sup>+0.4</sup><sub>-0.4</sub></td><td>3.5<sup>+0.4</sup><sub>-0.4</sub></td><td>4.7<sup>+1.3</sup><sub>-0.9</sub></td><td>2.3<sup>+0.6</sup><sub>-0.6</sub></td><td>2</td></tr><tr><td>eMACS0042.5-1103</td><td>0.5701</td><td>2.78</td><td>6.9 ± 2.0</td><td>11.9</td><td>822<sup>+210</sup><sub>-180</sub></td><td>4.5<sup>+0.6</sup><sub>-0.6</sub></td><td>5.9<sup>+0.8</sup><sub>-0.8</sub></td><td>9.2<sup>+5.3</sup><sub>-3.1</sub></td><td>5.9<sup>+5.8</sup><sub>-2.7</sub></td><td>4?</td></tr><tr><td>eMACS0045.2-0151</td><td>0.5471</td><td>3.02</td><td>5.2 ± 2.0</td><td>49.4</td><td>611<sup>+35</sup><sub>-33</sub></td><td>5.9<sup>+0.3</sup><sub>-0.3</sub></td><td>7.1<sup>+0.3</sup><sub>-0.3</sub></td><td>4.9<sup>+0.6</sup><sub>-0.5</sub></td><td>2.4<sup>+0.4</sup><sub>-0.4</sub></td><td>3</td></tr><tr><td>eMACS0135.2+0847</td><td>0.6185</td><td>5.95</td><td>7.9 ± 2.5</td><td>11.5</td><td>573<sup>+66</sup><sub>-54</sub></td><td>6.3<sup>+0.6</sup><sub>-0.6</sub></td><td>10.1<sup>+0.9</sup><sub>-0.9</sub></td><td>4.7<sup>+1.5</sup><sub>-0.9</sub></td><td>2.1<sup>+1.1</sup><sub>-0.6</sub></td><td>4</td></tr><tr><td>eMACS0248.2+0237</td><td>0.5561</td><td>6.10</td><td>6.6 ± 3.0</td><td>44.5</td><td>618<sup>+44</sup><sub>-35</sub></td><td>5.6<sup>+0.3</sup><sub>-0.3</sub></td><td>7.0<sup>+0.4</sup><sub>-0.3</sub></td><td>5.1<sup>+0.8</sup><sub>-0.6</sub></td><td>2.5<sup>+0.6</sup><sub>-0.4</sub></td><td>4</td></tr><tr><td>eMACS0256.7-1623</td><td>0.8621</td><td>4.81</td><td>19.6 ± 6.9</td><td>32.2</td><td>524<sup>+130</sup><sub>-88</sub></td><td>3.2<sup>+0.6</sup><sub>-0.4</sub></td><td>11.7<sup>+2.0</sup><sub>-1.6</sub></td><td>5.3<sup>+3.0</sup><sub>-1.6</sub></td><td>2.2<sup>+2.1</sup><sub>-0.9</sub></td><td>3</td></tr><tr><td>eMACS0256.9-1631</td><td>0.8670</td><td>4.86</td><td>18.3 ± 6.6</td><td>32.1</td><td>73<sup>+76</sup><sub>-49</sub></td><td>0.0<sup>+0.2</sup><sub>-0.0</sub></td><td>0.2<sup>+0.7</sup><sub>-0.7</sub></td><td>0.1<sup>+0.3</sup><sub>-0.3</sub></td><td>0.0<sup>+0.0</sup><sub>-0.0</sub></td><td>—</td></tr><tr><td>eMACS0256.9-1641</td><td>0.9023</td><td>13.94</td><td>14.4 ± 5.3</td><td>30.8</td><td>711<sup>+66</sup><sub>-49</sub></td><td>6.0<sup>+0.4</sup><sub>-0.4</sub></td><td>24.3<sup>+1.7</sup><sub>-1.6</sub></td><td>10.2<sup>+2.9</sup><sub>-1.8</sub></td><td>5.7<sup>+2.6</sup><sub>-1.4</sub></td><td>3</td></tr><tr><td>eMACS0324.0+2421</td><td>0.9023</td><td>13.94</td><td>14.4 ± 5.3</td><td>30.8</td><td>711<sup>+66</sup><sub>-49</sub></td><td>6.0<sup>+0.4</sup><sub>-0.4</sub></td><td>24.3<sup>+1.7</sup><sub>-1.6</sub></td><td>10.2<sup>+2.9</sup><sub>-1.8</sub></td><td>5.7<sup>+2.6</sup><sub>-1.4</sub></td><td>3</td></tr><tr><td>eMACS0305.2-9920</td><td>0.6028</td><td>1.31</td><td>6.7 ± 1.9</td><td>47.5</td><td>765<sup>+46</sup><sub>-43</sub></td><td>10.0<sup>+0.4</sup><sub>-0.4</sub></td><td>15.2<sup>+0.6</sup><sub>-0.5</sub></td><td>8.2<sup>+1.0</sup><sub>-0.9</sub></td><td>4.9<sup>+1.0</sup><sub>-0.8</sub></td><td>3</td></tr><tr><td>eMACS0804.6+5325</td><td>0.5786</td><td>4.29</td><td>5.8 ± 2.1</td><td>3.9</td><td>866<sup>+344</sup><sub>-172</sub></td><td>9.3<sup>+1.2</sup><sub>-1.3</sub></td><td>12.7<sup>+1.6</sup><sub>-1.6</sub></td><td>10.3<sup>+9.8</sup><sub>-3.7</sub></td><td>7.0<sup>+12.1</sup><sub>-3.4</sub></td><td>4?</td></tr><tr><td>eMACS0834.2+4524</td><td>0.6606</td><td>3.05</td><td>10.0 ± 2.8</td><td>32.6</td><td>640<sup>+27</sup><sub>-21</sub></td><td>14.6<sup>+0.5</sup><sub>-0.5</sub></td><td>27.5<sup>+0.9</sup><sub>-0.9</sub></td><td>6.2<sup>+0.9</sup><sub>-0.9</sub></td><td>3.1<sup>+0.9</sup><sub>-0.9</sub></td><td>4</td></tr><tr><td>eMACS0840.2+4421</td><td>0.6377</td><td>2.63</td><td>14.4 ± 4.3</td><td>31.4</td><td>683<sup>+45</sup><sub>-41</sub></td><td>8.6<sup>+0.4</sup><sub>-0.4</sub></td><td>14.9<sup>+0.7</sup><sub>-0.7</sub></td><td>6.9<sup>+0.9</sup><sub>-0.8</sub></td><td>3.7<sup>+0.8</sup><sub>-0.6</sub></td><td>2</td></tr><tr><td>eMACS0935.1+0614</td><td>0.7787</td><td>4.12</td><td>15.2 ± 4.8</td><td>29.9</td><td>680<sup>+54</sup><sub>-48</sub></td><td>9.3<sup>+0.5</sup><sub>-0.5</sub></td><td>26.3<sup>+1.3</sup><sub>-1.3</sub></td><td>8.1<sup>+1.3</sup><sub>-1.1</sub></td><td>4.3<sup>+1.1</sup><sub>-0.9</sub></td><td>3</td></tr><tr><td>eMACS1103.5+5132</td><td>0.5182</td><td>1.33</td><td>6.7 ± 1.4</td><td>19.7</td><td>593<sup>+21</sup><sub>-13</sub></td><td>13.1<sup>+0.3</sup><sub>-0.3</sub></td><td>13.7<sup>+0.4</sup><sub>-0.3</sub></td><td>4.5<sup>+0.3</sup><sub>-0.3</sub></td><td>2.1<sup>+0.2</sup><sub>-0.2</sub></td><td>1</td></tr><tr><td>eMACS1136.8+0005</td><td>0.5968</td><td>2.22</td><td>5.7 ± 2.0</td><td>3.9</td><td>735<sup>+129</sup><sub>-11</sub></td><td>6.4<sup>+1.1</sup><sub>-0.8</sub></td><td>9.4<sup>+1.2</sup><sub>-1.2</sub></td><td>7.6<sup>+2.9</sup><sub>-0.6</sub></td><td>4.4<sup>+0.2</sup><sub>-0.2</sub></td><td>4?</td></tr><tr><td>eMACS11144.2-2836</td><td>0.5070</td><td>7.30</td><td>16.5 ± 5.2</td><td>30.8</td><td>665<sup>+63</sup><sub>-64</sub></td><td>4.7<sup>+0.4</sup><sub>-0.4</sub></td><td>4.6<sup>+0.4</sup><sub>-0.4</sub></td><td>5.6<sup>+1.1</sup><sub>-0.1</sub></td><td>2.9<sup>+0.9</sup><sub>-0.8</sub></td><td>4</td></tr><tr><td>eMACS1157.9-1046</td><td>0.5570</td><td>3.44</td><td>7.6 ± 3.4</td><td>153.3</td><td>558<sup>+15</sup><sub>-15</sub></td><td>9.5<sup>+0.2</sup><sub>-0.2</sub></td><td>11.9<sup>+0.3</sup><sub>-0.3</sub></td><td>4.2<sup>+0.3</sup><sub>-0.2</sub></td><td>1.8<sup>+0.2</sup><sub>-0.2</sub></td><td>4</td></tr><tr><td>eMACS1209.4+2640</td><td>0.5553</td><td>1.58</td><td>6.1 ± 1.7</td><td>15.9</td><td>712<sup>+63</sup><sub>-52</sub></td><td>11.2<sup>+0.7</sup><sub>-0.7</sub></td><td>13.8<sup>+0.8</sup><sub>-0.8</sub></td><td>6.8<sup>+1.3</sup><sub>-0.6</sub></td><td>3.8<sup>+1.1</sup><sub>-0.6</sub></td><td>3</td></tr><tr><td>eMACS1341.9-2442</td><td>0.8339</td><td>7.41</td><td>16.2 ± 6.2</td><td>32.8</td><td>564<sup>+63</sup><sub>-50</sub></td><td>3.4<sup>+0.3</sup><sub>-0.2</sub></td><td>11.4<sup>+0.9</sup><sub>-0.8</sub></td><td>5.0<sup>+1.4</sup><sub>-1.0</sub></td><td>2.6<sup>+1.0</sup><sub>-0.6</sub></td><td>4</td></tr><tr><td>eMACS1350.7-1055</td><td>0.8247</td><td>4.17</td><td>12.0 ± 4.7</td><td>31.9</td><td>95<sup>+110</sup><sub>-27</sub></td><td>0.0<sup>+0.1</sup><sub>-0.0</sub></td><td>0.1<sup>+0.2</sup><sub>-0.1</sub></td><td>0.2<sup>+0.6</sup><sub>-0.1</sub></td><td>0.0<sup>+0.1</sup><sub>-0.0</sub></td><td>—</td></tr><tr><td>eMACS1353.7+4329</td><td>0.7364</td><td>1.17</td><td>9.0 ± 2.5</td><td>40.1</td><td>682<sup>+57</sup><sub>-57</sub></td><td>7.3<sup>+0.4</sup><sub>-0.4</sub></td><td>17.9<sup>+0.9</sup><sub>-0.9</sub></td><td>7.7<sup>+1.4</sup><sub>-1.1</sub></td><td>4.1<sup>+1.1</sup><sub>-0.3</sub></td><td>4</td></tr><tr><td>eMACS1407.0-0015</td><td>0.5520</td><td>4.15</td><td>6.0 ± 2.0</td><td>3.9</td><td>981<sup>+331</sup><sub>-353</sub></td><td>5.9<sup>+1.2</sup><sub>-1.2</sub></td><td>7.1<sup>+1.4</sup><sub>-1.4</sub></td><td>12.8<sup>+7.5</sup><sub>-10.1</sub></td><td>9.8<sup>+13.7</sup><sub>-7.2</sub></td><td>3?</td></tr><tr><td>eMACS1414.7+5446</td><td>0.6121</td><td>1.48</td><td>10.2 ± 0.6</td><td>32.6</td><td>842<sup>+65</sup><sub>-53</sub></td><td>10.9<sup>+0.5</sup><sub>-0.5</sub></td><td>17.0<sup>+0.8</sup><sub>-0.8</sub></td><td>10.1<sup>+1.6</sup><sub>-1.2</sub></td><td>6.7<sup>+1.7</sup><sub>-1.2</sub></td><td>3</td></tr><tr><td>eMACS1508.1+5755</td><td>0.5421</td><td>1.66</td><td>7.3 ± 1.5</td><td>47.3</td><td>800<sup>+57</sup><sub>-57</sub></td><td>7.8<sup>+0.3</sup><sub>-0.3</sub></td><td>9.1<sup>+0.4</sup><sub>-0.4</sub></td><td>8.4<sup>+1.2</sup><sub>-1.1</sub></td><td>5.3<sup>+1.2</sup><sub>-1.0</sub></td><td>4</td></tr><tr><td>eMACS1527.6+2044</td><td>0.6967</td><td>5.63</td><td>8.0 ± 3.6</td><td>24.5</td><td>734<sup>+52</sup><sub>-59</sub></td><td>9.4<sup>+0.6</sup><sub>-0.6</sub></td><td>20.3<sup>+1.3</sup><sub>-1.5</sub></td><td>8.5<sup>+2.0</sup><sub>-1.6</sub></td><td>4.9<sup>+1.8</sup><sub>-1.1</sub></td><td>4</td></tr><tr><td>eMACS1756.8+4008</td><td>0.5743</td><td>3.81</td><td>8.6 ± 1.8</td><td>126.1</td><td>699<sup>+20</sup><sub>-20</sub></td><td>16.2<sup>+0.3</sup><sub>-0.3</sub></td><td>21.8<sup>+0.4</sup><sub>-0.4</sub></td><td>6.7<sup>+0.4</sup><sub>-0.4</sub></td><td>3.7<sup>+0.3</sup><sub>-0.3</sub></td><td>4</td></tr><tr><td>eMACS1823.1+7822-NE</td><td>0.6803</td><td>3.89</td><td>†12.7 ± 1.9</td><td>22.1</td><td>496<sup>+38</sup><sub>-31</sub></td><td>5.0<sup>+0.3</sup><sub>-0.3</sub></td><td>9.9<sup>+0.6</sup><sub>-0.6</sub></td><td>3.8<sup>+0.6</sup><sub>-0.5</sub></td><td>1.5<sup>+0.4</sup><sub>-0.3</sub></td><td>1</td></tr><tr><td>eMACS1823.1+7822-SW</td><td>0.6784</td><td>3.89</td><td>†12.7 ± 1.9</td><td>22.1</td><td>528<sup>+65</sup><sub>-63</sub></td><td>2.5<sup>+0.3</sup><sub>-0.3</sub></td><td>4.9<sup>+0.5</sup><sub>-0.5</sub></td><td>4.2<sup>+1.1</sup><sub>-0.8</sub></td><td>1.8<sup>+0.7</sup><sub>-0.5</sub></td><td>3</td></tr><tr><td>eMACS1831.1+6214</td><td>0.8207</td><td>4.80</td><td>18.8 ± 2.2</td><td>22.0</td><td>713<sup>+65</sup><sub>-52</sub></td><td>11.5<sup>+0.5</sup><sub>-0.5</sub></td><td>36.7<sup>+2.0</sup><sub>-1.8</sub></td><td>9.2<sup>+1.3</sup><sub>-1.8</sub></td><td>5.2<sup>+1.1</sup><sub>-1.6</sub></td><td>4</td></tr><tr><td>eMACS1852.0+4900</td><td>0.6035</td><td>4.77</td><td>6.3 ± 1.7</td><td>19.8</td><td>596<sup>+68</sup><sub>-55</sub></td><td>8.4<sup>+0.8</sup><sub>-0.7</sub></td><td>12.7<sup>+1.2</sup><sub>-1.0</sub></td><td>5.0<sup>+1.2</sup><sub>-0.9</sub></td><td>2.3<sup>+0.9</sup><sub>-0.6</sub></td><td>4</td></tr><tr><td>eMACS12026.7-1920</td><td>0.6219</td><td>5.07</td><td>10.3 ± 3.4</td><td>31.8</td><td>540<sup>+28</sup><sub>-25</sub></td><td>6.1<sup>+0.3</sup><sub>-0.3</sub></td><td>9.9<sup>+0.5</sup><sub>-0.4</sub></td><td>4.2<sup>+0.4</sup><sub>-0.4</sub></td><td>1.8<sup>+0.2</sup><sub>-0.2</sub></td><td>1</td></tr><tr><td>eMACS2220.3-1211</td><td>0.5292</td><td>4.37</td><td>5.2 ± 2.0</td><td>12.4</td><td>923<sup>+112</sup><sub>-109</sub></td><td>10.0<sup>+0.9</sup><sub>-0.9</sub></td><td>11.0<sup>+0.9</sup><sub>-0.9</sub></td><td>11.0<sup>+2.9</sup><sub>-2.9</sub></td><td>8.0<sup>+1.0</sup><sub>-0.5</sub></td><td>2</td></tr><tr><td>eMACS2315.3-2128</td><td>0.5400</td><td>1.86</td><td>13.8 ± 5.4</td><td>30.8</td><td>925<sup>+273</sup><sub>-179</sub></td><td>3.1<sup>+0.4</sup><sub>-0.4</sub></td><td>3.6<sup>+0.5</sup><sub>-0.4</sub></td><td>11.2<sup>+7.6</sup><sub>-3.9</sub></td><td>8.1<sup>+9.5</sup><sub>-3.9</sub></td><td>3</td></tr><tr><td>eMACS2320.9+2912</td><td>0.4960</td><td>10.50</td><td>5.5 ± 1.6</td><td>9.6</td><td>698<sup>+113</sup><sub>-85</sub></td><td>14.6<sup>+1.4</sup><sub>-1.3</sub></td><td>13.8<sup>+1.4</sup><sub>-1.3</sub></td><td>6.1<sup>+2.1</sup><sub>-1.4</sub></td><td>3.3<sup>+1.9</sup><sub>-1.1</sub></td><td>3</td></tr><tr><td>eMACS2327.4-0204</td><td>0.7055</td><td>5.50</td><td>14.0 ± 4.1</td><td>163.6</td><td>710<sup>+16</sup><sub>-16</sub></td><td>17.9<sup>+0.3</sup><sub>-0.3</sub></td><td>39.7<sup>+0.7</sup><sub>-0.7</sub></td><td>8.0<sup>+0.4</sup><sub>-0.4</sub></td><td>4.5<sup>+0.3</sup><sub>-0.3</sub></td><td>2</td></tr></tbody></table>
<figure><img src="image_10.png" /></figure>

**Figure 9.** Bolometric luminosities and global ICM temperatures measured by us for the subsample of 35 eMACS clusters observed with CXO to date (red symbols), compared to the $L_X-kT$ values measured by Takey, Schwope & Lamer (2013) within $r_{500}$ for a sample of 345 clusters at $z = 0.03 - 0.67$ detected with XMM-Newton (blue symbols). The CXO temperatures were lowered by a factor of 0.889 to account for the known systematic offset between temperatures measured from data obtained by the two observatories (Schellenberger et al. 2015). Note that, unlike Takey and collaborators, we measured both quantities within $r_{1000}$.

### 5.3.1 ICM temperature and luminosity; cluster mass

Although the global cluster luminosities and ICM temperatures presented here should be considered tentative (our analysis is unable to account for low-surface-brightness emission from the cluster outskirts, or for cool cores, shock-heated gas, and any other temperature variations), the results are in good agreement with the $L_X-kT$ relation established for other X-ray-selected cluster samples (e.g. Takey et al. 2013, and references therein). As shown in Fig. 9, eMACS extends the $L_X-kT$ relation for galaxy clusters to higher luminosities and higher ICM temperatures than probed by existing X-ray-selected cluster samples at $z > 0.5$.

The results of our X-ray analysis summarized in Table 4 also unambiguously confirm that eMACS achieved its goal of probing both the redshift-X-ray luminosity and the redshift-mass regime highlighted in Fig. 1. As shown in Figs 4 and 9, the point-source-corrected X-ray luminosities of eMACS clusters are of the order of $10^{45}$ erg s$^{-1}$, consistent with the eMACS target range shown in Fig. 1 (left). The same holds for the cluster masses. Although the existing CXO observations allow direct measurements of the cluster mass (under the assumption of hydrostatic equilibrium) only to a radius of $r_{1000}$, $M_{200}$ can be estimated through extrapolation of the universal cluster mass profile to $r_{200}$ which approximately doubles these values (e.g. Pointecouteau et al. 2005; Ettori et al. 2019), yielding total masses in excess of $10^{15}$ M$_\odot$ for over one third of the subset for which CXO data are currently available (Fig. 10). We note that this extrapolation assumes the NFW profile found to describe the mass distribution in relaxed clusters (Navarro, Frenk & White 1997); for disturbed systems (of which eMACS found many; see below), the increase in the enclosed mass from $r_{1000}$ to $r_{200}$ would be even higher. Since our follow-up observations with CXO targeted preferentially the most distant and most X-ray luminous clusters, the mass distribution shown in Fig. 10 should, however, not be taken as representative of the eMACS sample as a whole.

<figure><img src="image_11.png" /></figure>

**Figure 10.** Distribution of total cluster masses ($M_{200}$) for the 33 eMACS clusters observed with CXO/ACIS-I to date (see Table 1). $M_{200}$ is estimated from $M_{1000}$ (Table 4) by extrapolating the universal cluster mass profile (e.g. Pointecouteau, Arnaud & Pratt 2005; Ettori et al. 2019) from $r_{1000}$ to $r_{200}$. The vertical line marks the mass of $10^{15}$ M$_\odot$ referred to in the Introduction and highlighted as characteristic for the eMACS target regime in mass-redshift space (Fig. 1, right).

### 5.3.2 Relaxation state

We show in Fig. 11 the X-ray surface brightness for all eMACS clusters with CXO/ACIS-I data. Note the wide range of morphologies, from almost perfectly circular and strongly peaked (e.g. eMACSJ1030.5 and eMACSJ2026.7) to extremely disturbed (e.g. eMACSJ1157.9, eMACSJ1508.1, and eMACSJ1756.8). Also noteworthy are two fields which show no diffuse ICM emission at all, eMACSJ0256.9 and eMACSJ1350.7; we discussed these eMACS misidentification earlier in Section 5.1.2 within the larger context of contamination from X-ray point sources.

Recognizing the limitations and pitfalls of a sophisticated ICM substructure analysis based on (typically) a few thousand photons, we adopt the simple but robust visual classification scheme of Ebeling et al. (2007) and assign each cluster a morphological classification from 1 (fully relaxed) through 4 (highly disturbed), considering both X-ray and optical appearance, as well as X-ray/optical alignment. The resulting classification, which has an uncertainty of about one morphology class, is indicated both in Fig. 11 and Table 4.

The low fraction of fully relaxed systems (morphological class 1) of ~10 per cent is immediately apparent, indicating that massive clusters at $z > 0.5$ are undergoing vigorous growth through the accretion of surrounding structures. At face value, the dominance of disturbed systems seems to contradict claims that X-ray-selected clusters samples suffer from a strong 'cool-core bias' in favour of relaxed clusters at higher redshift (Rossetti et al. 2017; Andrade-Santos et al. 2017). This claim is disputed though, as it may attribute erroneously to X-ray selection what is in fact caused by Malmquist bias: indeed, volume-complete (as opposed to flux-limited) samples of X-ray-selected clusters were found to be dominated not by cool-core clusters but by dynamically active, disturbed systems (Chon & Böhringer 2017). As is, the data shown in Fig. 11 cannot be used to assess the validity of either claim, since eMACS is not volume complete and since the subset of eMACS clusters with existing CXO data is biased in favour of disturbed systems simply as a result of the scientific goals of the underlying CXO proposals. A subset
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Contours of the adaptively smoothed X-ray surface brightness in the 0.5–7 keV band from the CXO observations described in Section 3.4. Contours in this and all subsequent figures are logarithmically spaced by factors of 1.2, starting at three times the background level (the latter varies around a median value of $3.4 \times 10^{-3}$ count s$^{-1}$ arcmin$^{-2}$). The 12 clusters previously discovered in the MACS project (Ebeling et al. 2007) are included only for completeness’ sake and shown in grey. The morphological class assigned to each cluster is shown in the upper right corner of each panel. (Note that we again exclude the 12 MACS clusters that, by design, are also part of the eMACS sample.)</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure 12.** As Fig. 1 (left) but focusing solely on eMACS and highlighting the subset of eMACS clusters observed with CXO/ACIS-I to date (coloured symbols denote different morphological classes). The 19 clusters with $L_{X,RASS} > 1.1 \times 10^{45}$ erg s$^{-1}$ (an additional two, shown as open circles, are the misidentifications discussed in Section 5.1.2), form an almost complete subsample of which only two are lacking CXO data. The 12 MACS clusters at $z > 0.5$ are shown as stars. Very few of the most X-ray luminous eMACS clusters exhibit a relaxed X-ray morphology.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 13.** The core of eMACSJ0252.4–2100, as viewed with HST; overlaid is the critical line (for a source at $z = 3.46$) from a lens model constrained by the two marked multiple-image systems (figure reproduced from Ebeling et al. 2021). Although no CXO data are available for eMACSJ0252 at present, the optical appearance and extreme dominance of the BCG suggest this system is very likely to have a morphological class of 1. (See also fig. B20.)</figcaption></figure>

with (almost) complete CXO coverage can, however, be defined by considering only the most X-ray luminous eMACS clusters. As shown in Fig. 12, only one of the 12 clusters in the almost volume-complete subsample of eMACS clusters with $L_{X,RASS} > 1.1 \times 10^{45}$ erg s$^{-1}$ is classified as fully relaxed (morphological class 1). Of the two systems without CXO data (both visible at $z \sim 0.7$ in Fig. 12), one (eMACSJ0252.4, shown in Fig. 13 and Fig. B20) can be safely

assumed to be highly relaxed (morphology class 1 or, at most, 2), based on the analysis of Ebeling et al. (2021) and the strong optical emission lines from its BCG, while the other (eMACSJ2025.5) is a very improbable candidate for the same classification, based on its optical appearance (see Fig. B83).

A conservative conclusion based on cluster X-ray morphology alone is thus that the fraction of cool-core clusters does not significantly exceed 10 per cent among the most X-ray luminous clusters at $z > 0.5$ (2 of 19), in agreement with the findings of Mann & Ebeling (2012). A decrease in the fraction of relaxed systems, as we approach the redshift of formation for massive clusters, is not only plausible on evolutionary grounds, but also expected from the impact of mergers in the rapidly evolving exponential tail of the most massive clusters. Such events boost, temporarily but dramatically, both the X-ray luminosity and the ICM temperature of the respective systems (Randall, Sarazin & Ricker 2002), thereby creating the opposite of a ‘cool-core bias’ (Fedeli & Bartelmann 2007). The fraction of cool-core systems identified by the presence of [OII]$\lambda$3727 line emission line in their BCG (18 of 110 in the full sample, or ~16 per cent) is, however, somewhat higher than the aforementioned conservative limit of 10 per cent and more consistent with the cool-core fraction of 10–20 per cent found both in cluster samples at lower redshift ($z < 0.3$; Edge, Stewart & Fabian 1992; Crawford et al. 1999; Green et al. 2016) based on the detection of $H\alpha$ in BCGs, when the relative strengths of these two lines are taken into account, and in SZ-selected cluster samples at $z > 0.5$ (McDonald et al. 2013; Ruppin et al. 2021).

In summary, the demographics of the eMACS sample and all observational evidence collected to date, perhaps surprisingly, do not suggest a marked change in the relative fractions of relaxation states of clusters at $z > 0.5$ compared to those in the local Universe.

## 5.4 The most X-ray luminous eMACS clusters

We here provide a brief overview highlighting the ten most X-ray luminous systems<fnref n="13" /> with $L_{X,CXO} \ge 1.5 \times 10^{45}$ erg s$^{-1}$, several of which were previously highlighted in Zalesky & Ebeling (2020), but note that an eleventh cluster, the massive merger eMACSJ0502.9 (Fig. B27), makes the same luminosity cut, too. A visual summary of this subsample is shown in Fig. 14; additional details for each cluster can be found in Appendix B. A more comprehensive analysis and discussion of the properties of the most X-ray luminous eMACS clusters<fnref n="14" /> will be given by Basto et al. (in preparation).

### 5.4.1 eMACSJ0324.0+2421

eMACSJ0324.0 deserves special mention for being the only galaxy cluster at $z > 0.9$ not merely detected, but in fact discovered in the RASS. Fig. B23 summarizes the observational evidence for this system (see also Fig. 14). Two almost equally luminous BCG

<footnote n="13">¹³These systems fall precisely into the extreme regime of the $L_{X-z}$ parameter space targeted by the eMACS project and highlighted in Fig. 1.</footnote>

<footnote n="14">¹⁴Note that the subset of the most X-ray luminous eMACS clusters selected from RASS data and highlighted in Fig. 12 differs from the one drawn from the CXO-based X-ray luminosities listed in Table 4 and considered here and by Basto et al. (in prep.). Although the X-ray fluxes recorded in the RASS and those derived from follow-up CXO observations are correlated, both photon noise and contamination from (possibly time variable) X-ray point sources introduce substantial scatter, as shown in Fig. 4. Interestingly, the subset of the most X-ray luminous eMACS clusters drawn from RASS data (Fig. 12) includes not only both of the eMACS misidentifications but also both of the most dramatic outliers from the mentioned correlation (see Fig. 4).</footnote>
<figure><img src="image_15.png" /><figcaption>**Figure 14.** HST images of the 10 most X-ray luminous eMACS clusters (based on point-source corrected fluxes measured from CXO observations). Overlaid in grey and (where lens models are available) yellow are isointensity contours of the adaptively smoothed X-ray emission and the critical line for gravitational lensing of a source at a fiducial redshift of $z = 2$, respectively.</figcaption></figure>
candidates, a disturbed ICM morphology that none the less features
only a single X-ray peak, a very high global gas temperature of
(13 ± 3) keV, a very large Einstein radius and very high enclosed
mass of M(< 240 kpc) = 4.6 × 10¹⁴ M☉ derived from a lens model
constrained by two spectroscopically confirmed multiple-image sys-
tems (see Section 5.2), and an exceptionally high velocity dispersion
of 1520<sup>+150</sup><sub>-220</sub> km s⁻¹ all suggest that eMACSJ0324.0 is an actively
growing, massive cluster, observed in the post-collision phase (but
still close to core passage) of a merger that proceeds largely along
our line of sight.

5.4.2 eMACSJ0834.2+4524

The appearance of eMACSJ0834.2 (Fig. B31, see also Fig. 14
and Section 5.2) provides ample and unambiguous evidence of an
ongoing collision of two components of comparable mass. Although
our strong-lensing mass model is unable to resolve multiple cluster-
scale components, two well separated peaks are observed in the X-ray
surface brightness. The offsets of these peaks in the ICM density from
the BCGs of the two subclusters are indicative of a collision axis that
lies at least partly in the plane of the sky. A line-of-sight component
of the merger-induced peculiar velocities is, however, also likely,
given the very high velocity dispersion of 1590<sup>+140</sup><sub>-200</sub> km s<sup>-1</sup>. We note
the quite modest ICM gas temperature of (6.2 ± 0.7) keV; such a
low global temperature could be indicative of the presence of cool
cores (the eastern component of e0834 exhibits a noticeably compact
X-ray peak) and points to the absence of large amounts of shock-
heated gas. We thus speculate that we either observe the merger well
before first core passage or that the collision involves a large impact
parameter, thereby leaving the cluster cores relatively unperturbed,
even after several core passages.

Owing to the system's highly disturbed morphology, our best-
fitting lens model is characterized by a large Einstein radius and a
high enclosed mass of M(< 220 kpc) = 3.2 × 10¹⁴ M☉.

5.4.3 eMACSJ0840.2+4421

While not fully relaxed (note the absence of a pronounced X-
ray peak and the non-circular geometry apparent in Fig. B32 and
Fig. 14), eMACSJ0840.2 is one of the least disturbed clusters in
this subset of the most X-ray luminous eMACS clusters. Both the
excellent X-ray/optical alignment and the substantial but (for a cluster
of this size) modest ICM temperature of (7.5 ± 0.7) keV suggest
that eMACSJ0840.2 is observed well past its most recent merger
event. Two spectroscopically confirmed multiple-image systems
constrain the mass in the cluster core to M(< 80 kpc) = 4 × 10¹³
M☉. Since the velocity dispersion of (1320 ± 120) km s⁻¹ (based
on 39 redshifts) of eMACSJ0840.2 is unlikely to be inflated by
ongoing merging activity along the line of sight, we can use the
virial scaling relation to compute an estimate of the dynamical mass
of the system (Evrard et al. 2008; Saro et al. 2013). The result,
Mdyn = (2.6 ± 0.1) × 10¹⁵ M☉, places eMACSJ0840.2 in the top
tier of massive clusters at any redshift (see Fig. 1).

5.4.4 eMACSJ0935.1+0614

Exceptionally X-ray luminous, in particular given its high redshift of
z = 0.78, eMACSJ0935.1 is one of two eMACS clusters in our top
10 list that do not currently have a lens model. The CXO data for this
system show neither a compact core nor pronounced substructure
in the ICM (Fig. 14); however, markedly elliptical X-ray surface-
brightness contours, as well as a clear offset of the X-ray peak from
the BCG, still identify eMACSJ0935.1 as a dynamically disturbed

cluster (Fig. B38). The cluster core features neither an extremely
luminous BCG nor signs of strong gravitational lensing, suggesting
a shallow mass profile that may be the result of mergers of several
clusters of moderate mass.

5.4.5 eMACSJ1353.7+4329

eMACSJ1353.7, previously mentioned in Section 5.2, has one of
the highest velocity dispersions of any eMACS cluster (see Sec-
tion 5.5.1); however, closer inspection of all available data (Fig.
B58) suggests that the measured value of 1600<sup>+110</sup><sub>-160</sub> km s<sup>-1</sup> is inflated
by an infalling group. The cluster exhibits the textbook signature (a
single X-ray peak between two BCGs, Fig. 14) of what is probably
a late-stage merger close to the plane of the sky. The two BCGs
are at essentially the same redshift (z = 0.736 and 0.739); the
looser group of galaxies to the east, however, features redshifts
close to z = 0.75, adding a pronounced and narrow peak to the
radial-velocity histogram that we interpret as evidence of infall of a
foreground group of galaxies. The lens model for eMACSJ1353.7,
well constrained by three multiple-image systems, one of which
is spectroscopically confirmed, yields a very high enclosed mass of
M(< 250 kpc) = 4.3 × 10¹⁴ M☉ for this powerful gravitational lens.
The not exceptionally high ICM temperature of (7.9 ± 1.2) keV mea-
sured by us suggests that this ongoing merger is observed well after
the temporary temperature boost triggered by the initial collision.

5.4.6 eMACSJ1414.7+5446

Like for eMACSJ1353.7, the extreme velocity dispersion measured
for eMACSJ1414.7 (1700<sup>+180</sup><sub>-210</sub> km s<sup>-1</sup>) is likely inflated by peculiar
velocities from an ongoing merger proceeding along an axis that
lies close to our line of sight. Although strong-lensing features are
clearly discernible near the cluster core (Fig. B60), we currently have
no spectroscopically confirmed multiple-image system that could
anchor a lens model for eMACSJ1414.7 The fact that the X-ray
peak of eMACSJ1414.7 is offset to the west of the BCG (Fig. 14),
with no obvious second cluster core apparent in this direction in the
galaxy distribution, might indicate a collision at a significant impact
parameter. The gap in the radial-velocity histogram and the high
global ICM temperature of 10 keV suggest that the eMACSJ1414.7
merger is either observed shortly after the first core passage or prior
to a second collision with an infalling group of galaxies.

5.4.7 eMACSJ1527.6+2044

eMACSJ1527.6 is another eMACS cluster with an exception-
ally high velocity dispersion, 1580<sup>+190</sup><sub>-230</sub> km s<sup>-1</sup>, but, in contrast to
eMACSJ1353.7 and eMACSJ1414.7, without clear signs of line-of-
sight substructure in the radial-velocity histogram (Fig. B69). Since
the ICM temperature of (8.7<sup>+1.9</sup><sub>-1.3</sub>) keV is above average, even for
massive clusters, we tentatively conclude that eMACSJ1527.6 is an
active merger observed during or shortly after first core passage.
Although we cannot rule out that the absence of multiple peaks in
the X-ray surface brightness distribution (Fig. 14) is primarily the
result of the very short CXO/ACIS-I exposure time of only 25 ks, the
presence of several BCG candidates in close proximity in the cluster
core suggests, in conjunction with the very velocity dispersion, that
the merging clusters are on trajectories that would make them difficult
to separate even in a much deeper X-ray observation. Strong-lensing
constraints from three multiple-image systems (one of them spectro-
scopically confirmed) yield a lens model of eMACSJ1527.6 that is
markedly more elongated than the ICM distribution (Fig. B69). A dis-
crepancy between the collisional and collisionless mass components
of this kind is a textbook characteristic of ongoing merger activity.
### 5.4.8 eMACSJ1756.8+4008

eMACSJ1756.8 is an obvious large-separation merger and the most powerful cluster lens in the eMACS sample according to ASTROLENS. Although we invested substantial resources into spectroscopic follow-up observations of the strong-lensing features identified in this system (Fig. 6 and Fig. B75), the lack of multiple-image systems near the eastern cluster core prevents us from constraining the mass in the presumably dominant part of this highly extended system (see also Section 5.2). As a result, the Einstein radius and enclosed mass listed for eMACSJ1756.8 in Table 3 should be considered firm lower limits. A very high total mass is expected also from the X-ray estimate of $5.4 \times 10^{14} M_{\odot}$ for the total cluster mass (Table 4) and the very high velocity dispersion of $1400^{+80}_{-90} \text{ km s}^{-1}$, which is based on 117 spectroscopic redshifts, the largest number available for any eMACS cluster. The wide (optical) separation of the three cluster cores in the plane of the sky (800 kpc altogether), the location of the X-ray peak between two of them, and the high ICM temperature of $(8.7 \pm 0.4)$ keV identify eMACSJ1756.8 unambiguously as a linear, post-collision merger of as many as three galaxy clusters.

### 5.4.9 eMACSJ1831.1+6214

Although narrowly beaten to the title of 'most X-ray luminous eMACS cluster' by eMACSJ2327.4 (see below), eMACSJ1831.1 ($L_{X,CXO} = (3.7 \pm 0.2) \times 10^{45} \text{ erg s}^{-1}$) stands out as one of only a handful exceptionally massive clusters known at $z > 0.8$. As is apparent from Fig. B78, eMACSJ1831.1 (not to be confused with eMACSJ1831.9, Fig. B79) is a highly complex system that consists of at least two components, as evinced by its very disturbed X-ray morphology and the presence of several optical cluster cores (Fig. 14). Both X-ray peaks are shallow and far from the closest BCG candidate, indicative of a pronounced segregation of collisional and non-collisional matter that is only observed in active mergers. Like eMACSJ1756.8 (Sections 5.2 and 5.4.8), eMACSJ1831.1 lacks prominent strong-lensing features near two of its apparent optical cores, causing the associated cluster-scale mass concentrations to be missing from our provisional LENSTOOL model. Spectroscopic confirmation of a faint straight arc that appears to mark a saddle point in the potential close to the NE X-ray peak (cf. Fig. B78) would help greatly to establish a lens model of this exceptional system. The complexity of eMACSJ1831.1 ($z = 0.8207$) is reflected also in redshift space, where the discovery of a superimposed background cluster at $z = 0.858$ suggests that the system is part of an even more extended large-scale structure.

### 5.4.10 eMACSJ2327.4-0204

The most X-ray luminous eMACS cluster, eMACSJ2327.4 (Fig. B98), was known previously and published as RCS2 J2327 by Gilbank et al. (2011). For completeness' sake we show in Fig. 14 (and in Fig. B98) the archival CXO data and a simple lens model based on the multiple-image systems at $z = 1.42$ and 2.98 (Fig. 6) identified by Sharon et al. (2015). Since RCS2 J2327 is a well known powerful gravitational lens, we refer the reader to the literature for details of its physical properties.

## 5.5 Cluster galaxy properties

The spectroscopic follow-up observations of galaxies in the fields of eMACS (candidate) clusters (see Section 3.3) served multiple purposes, the most fundamental of which was the confirmation of

the cluster ID and the establishment of a cluster redshift above the $z = 0.5$ threshold set for eMACS. Subsequent spectroscopic observations targeted not only the clusters' galaxy population in general (to measure velocity dispersions and search for substructure along the line of sight) but also specific galaxies of interest, such as the BCGs; possible cases of ongoing ram-pressure stripping of infalling galaxies; potential active galactic nucleus (AGN) within (or superimposed onto) the cluster flagged as X-ray point sources; and of course strong-lensing features (i.e. highly amplified background galaxies) whose redshifts are crucial to constrain the cluster mass distribution (Section 4.4).

Although an exhaustive discussion of the findings of all these studies is beyond the scope of this paper, we present brief summaries and highlights in the following sections.

### 5.5.1 Velocity dispersion and line-of-sight substructure

While a robust systemic cluster redshift can be derived from a handful of concordant redshifts, a credible velocity dispersion requires much better statistics and should ideally also be computed from a stellar-mass-limited and spatially complete sample, in order to avoid biases introduced by an over-representation of galaxies near the cluster core or, conversely, the cluster outskirts. Since our selection of galaxies for spectroscopic follow-up was largely driven by technical constraints (spectrograph characteristics) and our desire to maximize efficiency by simultaneously advancing all science goals served by these observations, the set of cluster galaxies with spectroscopic redshifts is statistically not well defined. Specifically, in clusters with few spectroscopic redshifts (fewer than 20) the dense cluster cores are likely to be undersampled, leading to potentially systematically low velocity dispersions.

We show in Fig. 15 the radial-velocity histograms of the 24 eMACS clusters for which at least 30 spectroscopic redshifts have been secured. At present, eMACSJ2229.9 is the only eMACS cluster for which multiple components along the line of sight can be clearly resolved with the radial-velocity data in hand. Fig. B93 summarizes our knowledge of this system, a superposition of two clusters at $z = 0.595$ and 0.625. No CXO data have been obtained for this system to date; however, the presence of RASS 6135 (Villforth et al. 2017), a luminous Quasi-Stellar Object (QSO) that is in fact a member of the $z = 0.625$ component of this double cluster, suggests a significant point-source contribution to the X-ray luminosity recorded in the RASS. A prominent fold arc at $z = 3.201$ with an obvious counterimage to the north, together with a second triple image at $z = 5.166$, constrain a highly elongated LENSTOOL model<fnref n="15" /> that, however, does not fully reflect the mass distribution of both cluster components acting in projection along our line of sight (Section 5.2 and Table 3).

<footnote n="15">¹⁵Our strong-lensing analysis supersedes that presented by Griffiths et al. (2021) which is based on an erroneous redshift for this fold arc (named 'Hamilton's Object' by them and dubbed 'The Scream' by us, in homage to Edvard Munch's famous painting). Our reanalysis of the spectral cube obtained of the arc by Griffiths and coworkers using the KCWI integral-field unit on Keck-2 confirms the presence of $Ly\alpha$, $Ly\beta$, the Lyman break, $Si\text{II}\lambda1260$, $O\text{I}\lambda1302$, and $C\text{II}\lambda1335$ ISM lines at $z = 3.20$ (these features had been identified by Griffiths et al. (2021) as $Mg\text{II}\lambda\lambda2796,2803$, $Fe\text{II}\lambda2365$, not noted, $Fe\text{II}\lambda2905$, $Fe\text{I}\lambda3004$, and $Ti\text{II}\lambda3074$, respectively, although many of the latter absorption features are typically very weak in the spectra of galaxies). These line identifications are in agreement with the redshift of $z = 3.201$ derived by us from the $[O\text{III}]\lambda\lambda 4959,5007$ emission lines in the Subaru/MOIRCS NIR spectrum shown in Fig. B93.</footnote>
<figure><img src="image_16.png" /><figcaption>**Figure 15.** Histograms of the radial velocities for eMACS clusters with at least 30 spectroscopic redshifts. We list for each cluster the velocity dispersion in units of km s⁻¹ and, in the upper left corner of each panel, the probability of the redshift histogram within δz = ±0.02 of the systemic cluster redshift being drawn from a Gaussian distribution, as well as the X-ray morphological class of each cluster from Fig. 11, where available.</figcaption></figure>

While the statistics of the radial-velocity data shown in Fig. 15 are too poor to clearly reveal line-of-sight substructure in any other eMACS cluster, the probability (as estimated by the Shapiro–Wilk statistic; Shapiro & Wilk 1965) of the observed set of redshifts having been drawn from a normal distribution is a useful diagnostic to complement the morphological classification based on a cluster's optical and X-ray appearance in projection on the sky (Section 5.3 and Fig. 11). As can be seen in Fig. 15, a disturbed morphology in the plane of the sky is not strongly correlated with significant substructure also along the line of sight: of the nine eMACS clusters in morphological class 3 or 4, only three have radial-velocity distributions that fail the Shapiro–Wilk normality test at greater than 95 per cent confidence ($p_N < 0.05$). Conversely, however, all eMACS clusters that fail the test and have been observed with CXO (eMACSJ1353.7, eMACSJ1508.1, and eMACSJ2320.9) are classified as heavily disturbed based on the X-ray/optical appearance, and the remaining two with $p_N < 0.05$ that have no CXO data as of yet (eMACSJ1709.5, Fig. B73, and eMACSJ1732.4, Fig. B74) show the same highly complex galaxy distribution associated with a morphological classification of 3 or 4. We tentatively conclude that the collisions in progress in these systems proceed along axes that are oblique with respect to both our line of sight and the plane of the sky. For the remaining eMACS clusters in morphology class 3 and 4, the good statistical agreement of the observed radial-velocity distributions with a Gaussian suggests that the respective merger activity occurs predominantly in the plane of the sky. These numbers are roughly consistent with isotropy in the orientation of the merger axis. Interestingly, the complementary (if small) subsample of the five eMACS clusters with at least 30 spectroscopically confirmed cluster members and a relaxed or only mildly disturbed morphology (morphology class 1 or 2) passes the Shapiro–Wilk normality test without exception.

Although encouraging, our insights into line-of-sight substructure remain qualitative even for the small subset of eMACS clusters

targeted most heavily in our spectroscopic follow-up observations. Much more extensive redshift surveys of almost all eMACS clusters and their surroundings will be required to map the environment in which eMACS clusters are embedded and the pathways along which they accrete the matter that fuels their growth.

### 5.5.2 Large-scale structure

On much larger scales than typically probed by our spectroscopic follow-up observations of individual clusters (see Section 3.3), a glimpse of the cosmic web (Bond, Kofman & Pogosyan 1996) is ironically provided with assistance from one of the non-clusters in the eMACS sample, eMACSJ0256.9. Although no diffuse X-ray emission is detected from eMACSJ0256.9 in our CXO follow-up observation (see Section 5.1.2), an overdensity of galaxies is clearly discernible both in our HST images and in the histogram of radial velocities of galaxies in this field (Fig. B22). We conclude that eMACSJ0256.9 is likely a filamentary structure aligned with our line of sight that masquerades as a three-dimensional overdensity only in this projection. Although eMACSJ0256.9 is thus not a three-dimensionally collapsed mass concentration, it appears to be a cluster in the process of formation, as indicated by the high velocity dispersion of $1170_{-240}^{+140}$ km s⁻¹ measured for galaxies in this structure. The presence of an actively evolving filament is of interest in the context of interconnected large-scale structure since eMACSJ0256.9–1631 ($z = 0.867$) is only 6 Mpc (13 arcmin) away in projection from eMACSJ0256.7–1623 ($z = 0.862$), an intrinsically massive cluster (Fig. B21). This separation amounts to only about three times the virial radius of eMACSJ0256.7, rendering matter overdensities within the putative massive filament candidates for accretion onto the node of the cosmic web marked by eMACSJ0256.7 within the next few Gyr.

While (after evaluation of the CXO data) eMACSJ0256.9–1631 and eMACSJ0256.7–1623 remain a pair in the eMACS catalogue
<figure><img src="image_17.png" /><figcaption>**Figure 16.** Close-up view (100 kpc on a side) of notable eMACS BCGs, as seen with *HST*. Evidence of dust obscuration is found in the majority of these galaxies. eMACSJ1057.5 and eMACSJ1148.0 show extended blue filaments to the north-east and south of the BCG, respectively, comparable to systems at lower redshift (O'Dea et al. 2010). All are detected in either our JVLA radio observations or VLASS data.</figcaption></figure>

only by virtue of the commonality of a high velocity dispersion, eMACSJ1823.1 ($z = 0.680$, Fig. B77) is a true cluster pair, and the only obvious pre-collision double cluster in our sample. Separated by 865 kpc in projection, both components appear dynamically relaxed, with the dominant north-eastern cluster exhibiting evidence of a cool core. Although the radial-velocity histograms show no sign of substructure along the line of sight (Fig. B77), the very high velocity dispersion of the NE component ($\sigma > 1500 \text{ km s}^{-1}$) suggests peculiar velocities (bulk motions) along the line of sight, a notion that is supported by the presence of a nearby foreground structure at $z = 0.65$. eMACSJ1823.1 thus marks a node of the cosmic web that might be actively accreting matter from orthogonal directions.

### 5.5.3 Brightest cluster galaxies

In addition to being prime targets for investigations of phenomena and physics operating on scales of hundreds of kpc, massive galaxy clusters provide a high-density environment that shapes the evolution of the constituent galaxies on kpc scales. An area of galaxy evolution that is of particular interest is the interaction of BCGs and their active nuclei with the surrounding ICM.

All X-ray-selected samples of galaxy clusters contain systems whose optical-light distribution is heavily dominated by the BCG; in such clusters, the BCG frequently displays strong optical emission lines, star formation, and an associated reservoir of cold molecular gas (e.g. Crawford et al. 1999; Green et al. 2016). As demonstrated in the Multi Unit Spectroscopic Explorer (MUSE) study of eMACSJ0252.4 (Ebeling et al. 2021, Fig. 13), such ‘active’ BCGs are seen out to $z \sim 0.7$ and stand out in the optical regime as bluer than the expected colour of an evolved stellar population.

Several BCGs in the eMACS sample in addition to eMACSJ0252.4 (Ebeling et al. 2021, Fig. 13) are notable in their optical morphology. Specifically, eMACS1030.5 (Fig. 16 and Fig. B42) harbours a spectacular case of an active BCG and is the brightest radio source in the eMACS sample, with properties comparable to those of compact, core-dominated sources such as 4C+55.16 (Hogan et al.

2015b; Rose et al. 2022; Timmerman et al. 2022) and MACSJ0242.5 (Allingham et al. 2023). Signs of active star formation, dust or multiple cores are discernible in the *HST* images of other eMACS BCGs, demonstrating that ‘active’ BCGs are common at $z > 0.5$; Fig. 16 shows 100 kpc × 100 kpc cutouts from our *HST* images for several examples.

Our spectroscopic follow-up covers all BCGs in eMACS clusters except for the one in eMACSJ1852.0 which at present has no publicly available spectrum. For 18 of the 110 BCGs in the sample, these spectra show significant [O II]λ3727 line emission (Fig. 17), indicating star formation, often comparable to that seen in the most active BCGs at lower redshifts (Green et al. 2016), where Hα line emission is typically used to gauge this activity. If we take the presence of an active BCG as an indicator of cluster relaxation, this spectroscopic estimate of the fraction of cool-core clusters of (15 ± 4) per cent is consistent with the percentage estimated from X-ray imaging (approximately 10 per cent) in Section 5.3. While the fraction of cool-core clusters is often found to be higher at lower redshifts, these more local studies benefit from both the accessibility of the Hα line and more extensive X-ray observations.

Radio emission is detected from 28 of the 66 BCGs observed at 5 GHz with the JVLA and from 31 of all 111 BCGs in the shallower VLASS 3 GHz data (see Table 1). The lower VLASS detection rate for the clusters not observed at 5 GHz suggests that our target selection for the JVLA observation was potentially biased towards radio emitters. Preferential selection of ‘active’ BCGs (Fig. 17) contributes to this bias, since the radio detection rate of BCGs is strongly correlated with the presence of optical line emission. Specifically, 16 of 18 (88 per cent) BCGs with [O II]λ3727 emission (Fig. 17) have radio detections, whereas only 15 of 92 (20 per cent) of the passive BCGs do. These detection fractions are consistent with the statistics of radio detections in X-ray-selected clusters at $z < 0.3$ from Hogan et al. (2015a), when matched to the 1.4-GHz radio-power limit of $3 \times 10^{23} \text{ W Hz}^{-1}$ reached for eMACS with VLASS data, of 80 per cent for line-emitting BCGs and 25 per cent for non-line emitting (see their fig. 14). This suggests little evolution in the radio power of line-emitting BCGs over the past ~6 Gyr.
<figure><img src="image_18.png" /><figcaption>**Figure 17.** Optical spectra of 17 of the 18 eMACS BCGs showing [O II]λ3727 line emission (the 18th one is the BCG of MACSJ1423). The respective host cluster is indicated above each panel; the spectral loci of atmospheric features are marked by vertical, shaded bands. Since the goal of our spectroscopic follow-up observations was solely the measurement of redshifts, no corrections for instrument response and no flux calibrations were applied to the obtained spectra. The spectrum of the BCG of eMACSJ0252 is truncated due to the red portion falling on a 'hot' CCD at the time of the DEIMOS observation; the impressive full spectrum (as obtained with MUSE) is shown in Ebeling et al. (2021). For the BCG of eMACSJ0840, we show the spectrum obtained with the blue arm of the LRIS spectrograph, which contains the [O II]λ3727 emission line; emission redward of about 6600 Å is suppressed by the dichroic.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure 18.** HST images (50 kpc on the side) of spectroscopically confirmed cluster members showing evidence of ram-pressure stripping by the ICM; the associated optical spectra are shown in Fig. 19. Shortened names in the upper left corner of each panel indicate the respective host cluster. Like for MACS (e.g. Ebeling et al. 2014; McPartland et al. 2016), additional cases of possible ram-pressure stripping are also seen in many other HST images of eMACS clusters but still await spectroscopic confirmation.</figcaption></figure>

### 5.5.4 Ram-pressure stripping

In addition to the physics of BCG evolution (see Section 5.5.3), the potential rapid morphological transformation of late-type field galaxies into early-type cluster members through ram-pressure stripping by the ICM (Gunn & Gott 1972) is another research topic that can be uniquely addressed through observations of massive X-ray luminous clusters.

While the textbook 'jellyfish' appearance of galaxies undergoing ram-pressure stripping (e.g. Cortese et al. 2007; Owers et al. 2012; Ebeling et al. 2014) is less readily discernible at the high redshifts of eMACS clusters, the superb angular resolution of HST reveals many promising cases in the eMACS sample. We show in Fig. 18 examples of spectroscopically confirmed cluster members likely experiencing ram-pressure stripping. Note that we include spirals with an almost undisturbed morphology since, in the environment encountered within massive clusters, essentially all late-type galaxies are expected to be undergoing ram-pressure stripping to some degree and will ultimately be stripped of the majority of their atomic and molecular gas. Many other late-type galaxies with similarly intriguing morphology are found in HST images of eMACS clusters and await spectroscopic follow-up observations to establish cluster membership.

Optical spectra of all galaxies shown in Fig. 18 are compiled in Fig. 19 and illustrate the diversity of the stellar populations of these sources. While a few exhibit spectra characterized by either purely passive evolution (as is the norm for early-type galaxies) or by pronounced star formation (as is typical for late-type systems), the majority are of composite type, most closely resembling post-starburst (E+A) galaxies (Couch & Sharples 1987; Dressler & Gunn 1992). These systems show strong hydrogen absorption features but only weak, or no, emission lines (specifically [O II]λ3727 and Hβ), a combination that is frequently observed in late-type and morphologically disturbed cluster members and attributed to ram-pressure stripping (e.g. Poggianti et al. 1999; Bekki 2014; French 2021; Werle et al. 2022).

Although their faintness and small angular size renders these galaxies challenging targets for in-depth follow-up studies of their internal dynamics, the excitation state of the intragalactic medium, or the effects of ram-pressure stripping on central black holes (all topics at the focus of 'jellyfish' studies in the more nearby Universe out to about $z = 0.3$: Kalita & Ebeling 2019, and references therein), the mere detection of debris trails or ICM-induced deformations of the galactic disc can provide valuable constraints on the direction of motion in the plane of the sky. The potential of ram-pressure stripping as a diagnostic of peculiar motions perpendicular to our line of sight was pointed out by Ebeling & Kalita (2019) and, in the eMACS sample, is exemplified by eMACSJ1757.5 (Fig. B76) which illustrates that the tell-tale morphology of 'jellyfish' galaxies can indeed reveal dominant axes of galaxy infall even at $z > 0.5$.

## 6 CONCLUSIONS

Based on RASS detections comprising as few as six photons, the eMACS has compiled a sample of over 100 very X-ray luminous galaxy clusters after searching over 20 000 deg² of extragalactic sky. Limited to clusters at $z > 0.5$, eMACS probes a population that was hitherto barely explored: the first generation of truly massive clusters ($M \gtrsim 10^{15} M_\odot$). Extensive follow-up observations of eMACS cluster candidates with ground- and space-based observatories, crucial to overcome the poor photon statistics of the underlying RASS detections, confirmed the presence of clusters in the vast majority of the targeted systems but, so far, have also shown two to be non-clusters. As importantly, follow-up work from radio to X-ray wavelengths yielded valuable insights into the physical properties of many of these systems and allowed us to constrain their gravitational-lensing strength, evolutionary state, and ICM gas temperature. In addition, these observations revealed examples of highly active BCGs, powerful QSOs, and late-type galaxies undergoing ram-pressure stripping during their high-velocity passage through the dense ICM.

The sample presented here includes the most distant galaxy cluster ever discovered in the RASS (eMACSJ0324.0 at $z = 0.90$, Fig. B23), extremely powerful gravitational lenses (e.g. eMACSJ1212.5, Fig. B51; eMACSJ1341.9, Fig. B56 and Ebeling et al. 2018; or eMACSJ1353.7, Fig. B58), a newly discovered 'exotic' hyperbolic umbilical lens (eMACSJ1248.2, Fig. 5), exceptionally X-ray luminous cluster collisions in various phases of the merging process (e.g. eMACSJ1756.8, Fig. B75; and eMACSJ1831.1, Fig. B78), extraordinarily 'active' BCGs, e.g. in eMACSJ0252.4 (Ebeling et al. 2021, Fig. 13) and eMACSJ1030.5 (Fig. 16), and numerous systems whose potentially remarkable physical properties are only hinted at by the existing data (e.g. eMACSJ1144.2, eMACSJ1508.1, and eMACSJ1757.5).
<figure><img src="image_20.png" /><figcaption>**Figure 19.** Optical spectra of the Ram Pressue Stripping (RPS) candidates shown in Fig. 18, in the same (RA) order. The coordinates of each galaxy are listed above each panel. Like in Fig. 17, the spectral loci of atmospheric features are marked by vertical, shaded bands. Spectra are truncated at 6500 Å if no useful redder data are available. Note that, while some RPS candidates exhibit classical early- or late-type spectra, the majority are approaching the E+A ('post-starburst') category, i.e. they feature pronounced Balmer absorption features but little or no emission lines indicative of ongoing star formation, as expected in this transition phase.</figcaption></figure>

We highlight in particular the potential importance of the massive clusters at $z > 0.5$ presented in this work as powerful gravitational telescopes to study faint sources in the very distant Universe (as already demonstrated, for example with the lensed sources at $z = 3.201$ and 5.166 in eMACSJ2229.9). The main benefits of selected eMACS clusters as gravitational magnifiers for *JWST* surveys of the very distant Universe, $z \gg 10$, are their high masses and high redshifts, which also yield a corresponding reduction in contamination from intracluster light, allowing faint lensed features to be more easily identified in the high-amplification regions near the cluster centres. We anticipate that *JWST* surveys of these clusters will be as productive as the current exploitation of the MACS sample for studying faint galaxies and individual stars in the high-redshift Universe (e.g. Abdurro'uf et al. 2023; Atek et al. 2023; Diego et al. 2023; Asada et al. 2024; Fujimoto et al. 2024).

While the current eMACS sample is not statistically complete, tentative conclusions about the physical properties of the population of very massive clusters at $z > 0.5$ can be drawn from better defined subsets. Specifically, the morphology of the ICM of eMACS clusters in high-resolution imaging X-ray observations and the optical

properties of eMACS BCGS from both imaging and spectroscopic follow-up work suggest that the cool-core fraction lies at or below ~15 per cent, consistent with the findings of Mann & Ebeling (2012) and expectations for a sample that approaches the redshift of cluster formation at $z \sim 1$. The analysis of all existing CXO/ACIS-I data for eMACS clusters shows that the power-law model of the $L_X-kT$ relation established for less massive clusters at lower redshifts (e.g. Takey et al. 2013) extends to the $z > 0.5$ regime probed by eMACS (Fig. 9). For the majority of eMACS clusters, velocity dispersions are very high, reaching and exceeding $1500 \text{ km s}^{-1}$ for several systems (Fig. 15). While clear signs of line-of-sight substructure are not detectable at the current statistics, the highest velocity dispersions exceed reasonable virial estimates and suggest that inflation from peculiar (bulk) velocities from ongoing cluster growth is common in this cluster population.

As, over three decades after its completion, the RASS is being succeeded by eRASS1 (Merloni et al. 2024), the second all-sky survey ever conducted with an imaging X-ray telescope, the samples of the most X-ray luminous clusters at $z > 0.3$ compiled by the MACS and eMACS projects are being complemented by a wealth
of discoveries down the mass and luminosity scale, all the way to groups of galaxies, protoclusters, and dense filaments. In addition, eROSITA's much greater sensitivity will yield vastly better photon statistics, and at least estimated ICM temperatures, for all eMACS clusters that, as of yet, have not been targeted in dedicated follow-up observations with CXO.

Although neither the compilation of the eMACS sample nor the analysis of the physical properties of all eMACS clusters are complete at this point, we here release all data and data products obtained and derived by us in the course of this programme to avoid duplication and unnecessary investments of precious observing resources by the extragalactic community (Zaznobin et al. 2023).

## ACKNOWLEDGEMENTS

We acknowledge comments from two referees and thank the second referee for constructive criticism and helpful suggestions. We thank Lukas Zalesky for permission to use unpublished ASTROLENS results in Section 5.2.2. of this paper and Ryan Cooke for help. HE gratefully acknowledges financial support from grants administered by STScI and Chandra X-ray Center (CXC). Specifically, support for programmes GO-13671, -14098, -15132, -15466, -15608, -15843, -15844, and -16428 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Associations of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Support for this work was also provided by the National Aeronautics and Space Administration through CXO Award Number GO9-20120X, GO0-21125X, GO1-22119X, and GO2-23119X, issued by the CXC, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. Durham authors acknowledge support from STFC (ST/T000244/1 and ST/X001075/1). This research has made use of data obtained from the Chandra Data Archive and the Chandra Source Catalogue, and software provided by the CXC in the application packages CIAO and SHERPA. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This research is based in part on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Based in part on observations obtained at the international Gemini Observatory, a programme of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation on behalf of the Gemini Observatory partnership: the U.S. National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministerio da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). This research has made use of the CIRADA cutout service at cutouts.cirada.ca, operated by the Canadian Initiative for Radio Astronomy Data Analysis (CIRADA). CIRADA is funded by a grant from the Canada Foundation for Innovation 2017 Innovation Fund (project 35999), as well as by the Provinces of Ontario, British Columbia, Alberta, Manitoba and Quebec, in collaboration with the National Research Council of Canada, the US National Radio Astronomy Observatory and Australia's Commonwealth Scientific and

Industrial Research Organisation. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made extensive use of NASA's Astrophysics Data System Bibliographic Services.

The authors wish to recognize and acknowledge the significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

## DATA AVAILABILITY

The data underlying this article are available in the MAST, CXO, JVLA, Keck, Subaru, Gemini, and ESO data archives.

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## SUPPORTING INFORMATION

Supplementary data are available at MNRAS online.

## Appendix B. Emacs One by One.

Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article.

## APPENDIX A: THE COMPILATION OF CLUSTER CANDIDATES FROM THE RASS

The eMACS project was launched in 2010, almost a decade before the launch of eROSITA, and was motivated by the realization that the RASS, the only existing all-sky survey with an imaging X-ray telescope, had not been fully exploited in search for massive clusters. In fact, there are thousands of X-ray sources below the flux limit of the MACS survey that can be examined in an even more ambitious cluster survey, and yet have never been tapped systematically. Our approach to using this valuable resource is detailed in the following.

### A1 The RASS Faint-Source Catalogue

Conducted in great circles through the ecliptic poles that result in a median exposure time of 370 s, the RASS was, until 2020, the only existing all-sky survey conducted in soft X-rays (0.1–2.4 keV) and a powerful resource for X-ray studies that require extremely large areal coverage. Two official source catalogues (the Bright Source Catalogue, BSC, and the Faint Source Catalogue, FSC) as well as

images and photon event tables for the entire survey are publicly available. The BSC contains only sources comprising at least 15 net photons, and featuring count rates in excess of 0.05 count s⁻¹; its defining criterion, however, is the maximum-likelihood value assigned to all RASS sources by the official RASS source detection algorithm. The BSC comprises 18 802 sources above this preset detection-likelihood threshold. MACS is based on sources selected from the BSC.

The much larger FSC, which lists over 100 000 additional sources comprising as few as six net photons, has seen very little use in scientific studies, due to its poorly known statistical properties. The enormous potential of the FSC becomes, however, immediately obvious when the distributions of net counts and count rate of FSC and BSC sources are inspected and compared. Following the approach taking for MACS (for details, see Ebeling et al. 2001), we limit both data sets to the extragalactic sky (|b| ≥ 20°) and the equatorial sky observable from Mauna Kea (−40° < δ − 80°) but apply no other cuts. The resulting histograms (proxies for the respective differential logN–logS distributions) for the BSC and FSC are plotted in Fig. A1 and demonstrate that

(i) in spite of its suggestive minimal source count value of 15, the BSC is not complete above this limit: 9 per cent of all RASS sources featuring more than 15 counts are in fact listed not in the BSC, but in the FSC;

(ii) for sources comprising between 6 and 11 photons, the FSC catalogue is about 20–30 per cent ‘overcomplete’, a well-known phenomenon caused by an increasing number of spurious sources (many of them multiple detections of very bright or very extended sources) as the photon statistics approach the ultimate detection limit of, roughly, 5–6 net counts;

(iii) when both catalogues are combined, the differential logN–logS distributions for the resulting data sets follow a power law well below the count limit of 17 adopted for MACS, and well below the count-rate limit of 0.05 count⁻¹ of the BSC in general.

Fig. A1 demonstrates that RASS-based surveys can be successfully extended significantly beyond the limits of the BSC, a claim that is supported by the fact that the slopes of about −2.3 of the best-fitting power laws describing the merged FSC/BSC data sets are in excellent agreement with the near-Euclidean value found for galaxy clusters and extragalactic RASS sources in general (Ebeling et al. 1998; Voges et al. 1999).

The conclusion that the RASS continues to hold great promise for cluster surveys may appear premature though, given that our analysis so far used all RASS sources in the extragalactic sky, i.e. a source population that is dominated by AGN and also includes a significant number of Galactic sources (primarily X-ray binaries and stars), all of which are X-ray point sources. Applying any insights gained from Fig. A1 to clusters would indeed be impermissible if we aimed to find additional X-ray-faint systems at low redshift,<fnref n="16" /> where even galaxy groups exhibit significant angular extent in the RASS. At $z > 0.4$, however, galaxy clusters appear almost point-like at the angular resolution of the RASS (Fig. A2). As a result, the background in the standard RASS detect cell is negligible for sources comprising at least 10 net photons, except for the deepest part of the RASS near the ecliptic poles where integration times approaching, and over a small solid angle in fact exceeding, 1000 s are reached, resulting in increased background counts and thus a higher effective detection threshold in terms of net source counts (Voges et al. 2001).

<footnote n="16">¹⁶And thus also of low X-ray luminosity.</footnote>
<figure><img src="image_21.png" /><figcaption>**Figure A1.** Differential logN–logS distributions for net counts (left) and count rates (right) of BSC (orange) and FSC (shaded) sources at $|b| \ge 20^\circ$. For the count distribution (top) only sources featuring exposure times between 200 and 300 s were considered, thereby mitigating the impact of exposure time variations on the RASS detection efficiency. The source count limit of 17 photons adopted during the compilation of the earlier MACS sample is marked by the dashed vertical line. The green histograms show the distributions that result when both catalogues are combined; best-fitting power laws are shown in red.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Figure A2.** Left: cumulative distribution of source extent likelihood in the RASS for three classes of sources: stars (as a reference for point sources), nearby clusters, and distant clusters. The extent likelihood distribution for stars illustrates that values below 5 (dotted vertical line) are not significant and often spurious. Note how over 60 per cent of all distant clusters are classified as point sources. Right: cumulative distribution of source extent for the same three source classes as shown on top, but using only sources with extent likelihood values of at least 5. The dotted vertical line marks the FWHM of the RASS PSF, which is also the pixel size of the RASS. Even for the minority of all distant clusters classified as extended (left panel) the assigned angular extent exceeds 1 pixel only for fewer than 40 per cent.</figcaption></figure>

Although the efficiency of the RASS detection algorithm near the detection limit has to be carefully tested as a function of RASS exposure time, the potential rewards make this effort very worthwhile. We show, in Fig. A3, the differential logN–logS distributions in terms of energy flux<fnref n="17" /> when two different source count thresholds are applied: the top panel illustrates the impact of the FSC for sources with net counts in excess of 17 photons, the limit adopted for MACS. Not surprisingly, little can be gained above the MACS flux limit of

$1 \times 10^{-12} \text{ erg cm}^{-2} \text{ s}^{-1}$, but at fluxes down to $0.5 \times 10^{-12} \text{ erg cm}^{-2} \text{ s}^{-1}$ the inclusion of the FSC improves upon the MACS sample size by 66 per cent. If the count limit is lowered to 12 (bottom panel of Fig. A3), easily achievable as shown in Fig. A1, the combined FSC/BSC data set increases the survey's effective solid angle noticeably already within the MACS regime (i.e. at $f_X > 1 \times 10^{-12} \text{ erg cm}^{-2} \text{ s}^{-1}$), and allows the compilation of a sample of more than twice the size of MACS at $f_X > 0.5 \times 10^{-12} \text{ erg cm}^{-2} \text{ s}^{-1}$.

The cluster sample presented in this paper adopts a flux limit of $f_X > 0.5 \times 10^{-12} \text{ erg cm}^{-2} \text{ s}^{-1}$ (0.1–2.4 keV) and the same X-ray spectral-hardness threshold as MACS (see Ebeling et al. 2001). Since our goal is not statistical completeness, but the compilation of the largest-possible RASS-selected cluster sample, the additional criterion of 12 net RASS photons from Fig. A3 was not enforced. However, over 80 per cent of the sample reach a count of 10. An

<footnote n="17">¹⁷For the conversion from count rate to flux, we assumed a spectrum typical of very X-ray luminous clusters at moderately high redshift, characterized by $z = 0.2$, $kT = 8$ keV, $Z = 0.2$, and the Galactic $n_H$ column density in the direction of the source. Modifying these assumptions does not affect any of our conclusions noticeably.</footnote>
<figure><img src="image_23.png" /></figure>

**Figure A3.** Differential logN–logS distributions for BSC (orange) and FSC (shaded) sources at $|b| \ge 20^\circ$. The plot on the left uses only sources comprising at least 17 net photons (the threshold used by the earlier MACS project and marked in Fig. 3); for the right-hand plot this limit is lowered to 12 photons. Green histograms and red power-law models refer to the distributions resulting when the FSC and BSC catalogues are merged.

<figure><img src="image_24.png" /></figure>

**Figure A4.** PS1/3π image (gri) of a 5 × 5 arcmin⁻² region centred on the RASS source 1RXS J175651.8+400802. An overdensity of red galaxies near the image centre is immediately visible, as is a second apparent cluster core about 2 arcmin to the west.

overview of the RASS fluxes, exposure times, source likelihoods,<fnref n="18" /> and photon counts of the eMACS sample is provided in Table A1. Note that the subset of eMACS clusters observed with CXO is representative of the full sample, in the sense that both data sets span the same range (and have the same median) in LH<sub>RASS</sub> and n<sub>ph,RASS</sub>. Moreover, CXO confirmed (see Table 4) both the systems with the highest n<sub>ph,RASS</sub> value (eMACSJ1831.1; n<sub>ph,RASS</sub> = 125) and the one with the second-lowest (eMACSJ0248.2, n<sub>ph,RASS</sub> = 6).

## A2 Optical screening

Our strategy for the identification of distant galaxy clusters from these data sets is brute force: we select all X-ray sources listed in the RASS BSC and FSC that fall within our study area and above the thresholds in X-ray flux and spectral hardness ratio, and then examine PS1 images in the g, r, i, and z bands in a 5 × 5 arcmin² region around the X-ray source position. Our goal, however, is not the identification of every X-ray source in the BSC and FSC; far from it. For each RASS source, we merely seek to answer the much simpler question: ‘Based on the colour images provided by PS1, could the optical counterpart of this source conceivably be a massive, distant cluster?’ In essence this entails the visual identification of candidate clusters at $z \gtrsim 0.3$ as overdensities of faint, red galaxies, a simple approach that is motivated by the success of MACS, which demonstrated that distant RASS-detected clusters are readily discernible in relatively shallow optical images. As illustrated by Fig. A4, which shows eMACSJ1756.8 ($z = 0.574$) as viewed by PS1 in the 3π survey (see also Section 5.4.8), this strategy remains successful for the population of X-ray fainter clusters probed by eMACS.

At the highest redshifts probed by eMACS, however, the Balmer/4000 Å break in cluster galaxies is getting close to moving out of the optical window. Care must thus be taken not to reject a RASS source based on the absence of an obvious galaxy overdensity in a gri PS1 image; unless the presence of a bright star, a QSO candidate, or some other, known celestial source<fnref n="19" /> suggest a non-cluster identification, a ‘blank field’ in a PS1/3π image is kept in our list of candidates and assigned high priority for follow-up observation. Although the associated follow-up observations resulted, in most cases, in the rejection of such sources, several discoveries vindicate our conservative approach, as is illustrated by Fig. A5 which shows an apparently blank field at the location of a RASS source. Owing to the lack of a plausible non-cluster identification we none the less included this source in our list of eMACS cluster candidates, a decision that led to the discovery of eMACSJ0324.0 ($z = 0.90$) (see Section 5.4.1).

<footnote n="18">¹⁸A detailed description of the maximum-likelihood algorithm used in the processing of RASS data can be found in Cruddace, Hasinger & Schmitt. (1987) and Boese & Doebereiner (2001).</footnote>

<footnote n="19">¹⁹In order to aid the identification of non-cluster sources, we also query NASA’s Extragalactic Database (NED) for known celestial sources within 2 arcmin radius of the respective X-ray source.</footnote>
<figure><img src="image_25.png" /><figcaption>**Figure A5.** PS1/3π image of a 5 × 5 arcmin⁻² region centred on the RASS source 1RXS J032401.8+242131. Requiring cluster candidates to show a pronounced galaxy overdensity in a *gri* image (left) would have led to the elimination of this source; even in the *riz* image (right) the cluster galaxies are barely visible. Follow-up observations unambiguously established the presence of a massive galaxy cluster at z = 0.9, the most distant cluster discovered to date in the RASS.</figcaption></figure>

<table><caption>**Table A1.** RASS detection parameters for the eMACS sample: flux (in the 0.1–2.4 keV band) within the detect cell, exposure time, source likelihood (Cruddace et al. 1987; Boese & Doebereiner 2001), and net number of photons. The RASS flux of eMACSJ1419.2+5326 falls below 0.5 × 10⁻¹² erg s⁻¹ cm⁻²; we none the less include the system here as a discovery from the eMACS pilot project (Ebeling et al. 2013), which did not impose the flux limit adopted later for eMACS.</caption><thead><tr><th>Name</th><th>f<sub>RASS</sub><br>(10<sup>-12</sup> erg s<sup>-1</sup>)</th><th>t<sub>RASS</sub><br>(s)</th><th>LH<sub>RASS</sub></th><th>n<sub>ph,RASS</sub></th><th>Name</th><th>f<sub>RASS</sub><br>(10<sup>-12</sup> erg s<sup>-1</sup>)</th><th>t<sub>RASS</sub><br>(s)</th><th>LH<sub>RASS</sub></th><th>n<sub>ph,RASS</sub></th></tr></thead><tbody><tr><td>eMACSJ0014.9+0056</td><td>0.71</td><td>382</td><td>14</td><td>16.0</td><td>eMACSJ1222.3+2418</td><td>0.98</td><td>462</td><td>40</td><td>31.0</td></tr><tr><td>MACSJ0018.5+1626</td><td>1.33</td><td>246</td><td>27</td><td>18.0</td><td>eMACSJ1248.2+0743</td><td>0.50</td><td>376</td><td>11</td><td>13.2</td></tr><tr><td>MACSJ0025.4+1222</td><td>1.09</td><td>363</td><td>21</td><td>24.3</td><td>eMACSJ1251.3+3131</td><td>0.56</td><td>516</td><td>25</td><td>22.2</td></tr><tr><td>eMACSJ0026.2+0120</td><td>0.72</td><td>501</td><td>32</td><td>22.0</td><td>eMACSJ1257.3+3654</td><td>0.70</td><td>531</td><td>35</td><td>28.1</td></tr><tr><td>eMACSJ0027.0+1252</td><td>0.60</td><td>313</td><td>12</td><td>9.7</td><td>eMACSJ1341.9+2442</td><td>0.70</td><td>291</td><td>8</td><td>10.2</td></tr><tr><td>eMACSJ0030.5+2618</td><td>0.61</td><td>346</td><td>8</td><td>11.8</td><td>eMACSJ1350.7+1055</td><td>0.53</td><td>357</td><td>12</td><td>10.4</td></tr><tr><td>eMACSJ0031.2+1907</td><td>0.51</td><td>354</td><td>9</td><td>9.9</td><td>eMACSJ1353.7+4329</td><td>0.51</td><td>654</td><td>19</td><td>25.5</td></tr><tr><td>eMACSJ0042.5+1103</td><td>0.68</td><td>355</td><td>22</td><td>15.3</td><td>eMACSJ1407.0+0015</td><td>0.64</td><td>403</td><td>17</td><td>14.9</td></tr><tr><td>eMACSJ0043.1+1129</td><td>0.52</td><td>366</td><td>17</td><td>12.4</td><td>eMACSJ1414.7+5446</td><td>0.87</td><td>654</td><td>23</td><td>43.8</td></tr><tr><td>eMACSJ0045.2+0151</td><td>0.56</td><td>316</td><td>11</td><td>10.4</td><td>eMACSJ1419.1+0624</td><td>0.51</td><td>319</td><td>9</td><td>9.6</td></tr><tr><td>eMACSJ0101.1+1831</td><td>0.52</td><td>429</td><td>19</td><td>12.4</td><td>eMACSJ1419.2+5326</td><td>0.26</td><td>638</td><td>11</td><td>13.4</td></tr><tr><td>eMACSJ0121.4+2143</td><td>0.53</td><td>381</td><td>10</td><td>10.7</td><td>MACSJ1423.8+2404</td><td>1.03</td><td>450</td><td>32</td><td>29.2</td></tr><tr><td>eMACSJ0122.7+1454</td><td>0.75</td><td>185</td><td>8</td><td>7.6</td><td>eMACSJ1430.0+4127</td><td>0.66</td><td>630</td><td>33</td><td>32.1</td></tr><tr><td>eMACSJ0124.0+0430</td><td>1.21</td><td>428</td><td>10</td><td>29.5</td><td>eMACSJ1437.8+0616</td><td>0.65</td><td>281</td><td>17</td><td>11.5</td></tr><tr><td>eMACSJ0127.5+0606</td><td>0.68</td><td>425</td><td>17</td><td>15.7</td><td>eMACSJ1443.2+0102</td><td>0.50</td><td>344</td><td>7</td><td>10.0</td></tr><tr><td>eMACSJ0135.2+0847</td><td>0.66</td><td>416</td><td>21</td><td>14.1</td><td>eMACSJ1508.1+5755</td><td>0.81</td><td>876</td><td>11</td><td>50.8</td></tr><tr><td>eMACSJ0153.4+1722</td><td>1.08</td><td>103</td><td>8</td><td>5.8</td><td>eMACSJ1518.3+2927</td><td>0.59</td><td>381</td><td>22</td><td>14.5</td></tr><tr><td>eMACSJ0200.3+2454</td><td>0.51</td><td>489</td><td>19</td><td>18.6</td><td>eMACSJ1523.1+1329</td><td>0.57</td><td>396</td><td>9</td><td>13.5</td></tr><tr><td>eMACSJ0224.5+1615</td><td>0.57</td><td>377</td><td>14</td><td>13.6</td><td>eMACSJ1527.6+2044</td><td>0.51</td><td>280</td><td>8</td><td>7.6</td></tr><tr><td>eMACSJ0241.0+2557</td><td>0.85</td><td>352</td><td>18</td><td>13.4</td><td>eMACSJ1621.2+0231</td><td>0.53</td><td>463</td><td>10</td><td>10.6</td></tr><tr><td>eMACSJ0248.2+0237</td><td>0.69</td><td>174</td><td>11</td><td>6.1</td><td>eMACSJ1621.4+7232</td><td>0.56</td><td>1623</td><td>60</td><td>51.9</td></tr><tr><td>eMACSJ0252.4+2100</td><td>0.79</td><td>237</td><td>18</td><td>11.9</td><td>eMACSJ1631.1+1528</td><td>0.59</td><td>211</td><td>8</td><td>6.5</td></tr><tr><td>eMACSJ0256.7+1623</td><td>0.80</td><td>260</td><td>19</td><td>11.7</td><td>eMACSJ1709.5+4731</td><td>0.64</td><td>851</td><td>46</td><td>34.9</td></tr><tr><td>eMACSJ0256.9+1631</td><td>0.73</td><td>259</td><td>15</td><td>11.1</td><td>eMACSJ1732.4+1934</td><td>0.52</td><td>605</td><td>10</td><td>15.7</td></tr><tr><td>MACSJ0257.1+2325</td><td>1.23</td><td>289</td><td>36</td><td>24.0</td><td>eMACSJ1756.8+4008</td><td>0.84</td><td>915</td><td>17</td><td>44.8</td></tr><tr><td>eMACSJ0324.0+2421</td><td>0.52</td><td>455</td><td>20</td><td>10.0</td><td>eMACSJ1757.5+3045</td><td>0.52</td><td>764</td><td>9</td><td>20.6</td></tr><tr><td>eMACSJ0325.4+0359</td><td>0.58</td><td>323</td><td>10</td><td>10.3</td><td>eMACSJ1823.1+7822</td><td>0.87</td><td>1529</td><td>31</td><td>73.4</td></tr><tr><td>eMACSJ0403.6+1544</td><td>0.85</td><td>290</td><td>13</td><td>8.7</td><td>eMACSJ1831.1+6214</td><td>0.85</td><td>2768</td><td>89</td><td>124.6</td></tr><tr><td>eMACSJ0429.0+1011</td><td>0.57</td><td>238</td><td>7</td><td>6.7</td><td>eMACSJ1831.9+5746</td><td>0.61</td><td>1566</td><td>48</td><td>50.1</td></tr><tr><td>MACSJ0454.1+0300</td><td>1.55</td><td>381</td><td>50</td><td>30.1</td><td>eMACSJ1852.0+4900</td><td>0.54</td><td>808</td><td>22</td><td>23.4</td></tr><tr><td>eMACSJ0502.9+2902</td><td>0.58</td><td>490</td><td>19</td><td>20.6</td><td>eMACSJ2018.3+2242</td><td>0.52</td><td>354</td><td>9</td><td>8.9</td></tr><tr><td>MACSJ0647.7+7015</td><td>1.34</td><td>418</td><td>27</td><td>28.8</td><td>eMACSJ2020.1+1432</td><td>0.54</td><td>317</td><td>9</td><td>8.9</td></tr><tr><td>MACSJ0717.5+3745</td><td>1.92</td><td>459</td><td>68</td><td>42.2</td><td>eMACSJ2025.5+2313</td><td>0.86</td><td>193</td><td>8</td><td>8.3</td></tr><tr><td>eMACSJ0742.9+5001</td><td>0.66</td><td>384</td><td>20</td><td>12.7</td><td>eMACSJ2026.7+1920</td><td>0.85</td><td>340</td><td>10</td><td>15.3</td></tr><tr><td>MACSJ0744.8+3927</td><td>1.19</td><td>450</td><td>45</td><td>27.0</td><td>eMACSJ2037.1+2534</td><td>0.63</td><td>295</td><td>11</td><td>9.7</td></tr></tbody></table>
<table><caption>Table A1 – continued</caption><thead><tr><th>Name</th><th>f<sub>RASS</sub><br>(10<sup>-12</sup> erg s<sup>-1</sup>)</th><th>t<sub>RASS</sub><br>(s)</th><th>LH<sub>RASS</sub></th><th>n<sub>ph,RASS</sub></th><th>Name</th><th>f<sub>RASS</sub><br>(10<sup>-12</sup> erg s<sup>-1</sup>)</th><th>t<sub>RASS</sub><br>(s)</th><th>LH<sub>RASS</sub></th><th>n<sub>ph,RASS</sub></th></tr></thead><tbody><tr><td>eMACSJ0747.0+6937</td><td>0.56</td><td>293</td><td>10</td><td>9.4</td><td>eMACSJ2045.8-2438</td><td>0.80</td><td>219</td><td>10</td><td>9.2</td></tr><tr><td>eMACSJ0804.6+5325</td><td>0.55</td><td>424</td><td>11</td><td>12.7</td><td>eMACSJ2100.4+0724</td><td>0.77</td><td>464</td><td>24</td><td>17.6</td></tr><tr><td>eMACSJ0834.2+4524</td><td>0.71</td><td>438</td><td>32</td><td>18.4</td><td>eMACSJ2114.4+0427</td><td>0.66</td><td>460</td><td>19</td><td>14.7</td></tr><tr><td>eMACSJ0840.2+4421</td><td>1.13</td><td>226</td><td>14</td><td>15.6</td><td>MACSJ2129.4-0741</td><td>1.05</td><td>375</td><td>30</td><td>20.2</td></tr><tr><td>eMACSJ0841.8-0429</td><td>0.92</td><td>306</td><td>24</td><td>16.8</td><td>eMACSJ2137.2-2232</td><td>0.70</td><td>344</td><td>16</td><td>13.8</td></tr><tr><td>eMACSJ0850.2-0611</td><td>0.52</td><td>325</td><td>13</td><td>10.1</td><td>eMACSJ2201.9+0711</td><td>0.63</td><td>226</td><td>7</td><td>7.0</td></tr><tr><td>eMACSJ0910.8+3850</td><td>0.63</td><td>376</td><td>15</td><td>16.5</td><td>eMACSJ2207.5-0302</td><td>0.70</td><td>274</td><td>14</td><td>9.6</td></tr><tr><td>MACSJ0911.2+1746</td><td>1.05</td><td>368</td><td>30</td><td>21.7</td><td>MACSJ2214.9-1359</td><td>1.42</td><td>283</td><td>28</td><td>23.5</td></tr><tr><td>eMACSJ0921.6-0621</td><td>0.55</td><td>268</td><td>7</td><td>8.6</td><td>eMACSJ2220.3-1211</td><td>0.60</td><td>292</td><td>12</td><td>9.6</td></tr><tr><td>eMACSJ0934.6+0540</td><td>1.05</td><td>369</td><td>13</td><td>21.8</td><td>eMACSJ2229.9-0808</td><td>0.67</td><td>233</td><td>7</td><td>8.4</td></tr><tr><td>eMACSJ0935.1+0614</td><td>0.77</td><td>374</td><td>10</td><td>16.1</td><td>eMACSJ2236.0+3451</td><td>0.57</td><td>528</td><td>19</td><td>12.7</td></tr><tr><td>eMACSJ0943.3-1842</td><td>0.64</td><td>472</td><td>20</td><td>15.6</td><td>eMACSJ2315.3-2128</td><td>1.57</td><td>88</td><td>12</td><td>9.2</td></tr><tr><td>eMACSJ1025.0-1354</td><td>0.62</td><td>396</td><td>19</td><td>11.5</td><td>eMACSJ2316.6+1246</td><td>0.57</td><td>444</td><td>19</td><td>13.8</td></tr><tr><td>eMACSJ1027.2+0345</td><td>0.53</td><td>313</td><td>8</td><td>9.4</td><td>eMACSJ2320.9+2912</td><td>0.73</td><td>469</td><td>21</td><td>16.9</td></tr><tr><td>eMACSJ1030.5+5132</td><td>0.82</td><td>547</td><td>41</td><td>33.9</td><td>eMACSJ2327.4-0204</td><td>0.88</td><td>361</td><td>24</td><td>17.0</td></tr><tr><td>eMACSJ1050.6+3548</td><td>0.54</td><td>312</td><td>13</td><td>11.5</td><td>eMACSJ2351.6-2818</td><td>0.64</td><td>311</td><td>16</td><td>14.3</td></tr><tr><td>eMACSJ1057.5+5759</td><td>0.77</td><td>406</td><td>15</td><td>28.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1131.1+3553</td><td>0.52</td><td>259</td><td>10</td><td>9.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1136.8+0005</td><td>0.50</td><td>434</td><td>12</td><td>13.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1144.2-2836</td><td>2.16</td><td>140</td><td>9</td><td>14.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1148.0+5116</td><td>0.71</td><td>390</td><td>23</td><td>19.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MACSJ1149.5+2223</td><td>1.20</td><td>460</td><td>35</td><td>35.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1157.9-1046</td><td>0.79</td><td>157</td><td>12</td><td>6.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1209.4+2640</td><td>0.63</td><td>467</td><td>24</td><td>20.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>eMACSJ1212.5-1216</td><td>0.57</td><td>406</td><td>19</td><td>12.2</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

## APPENDIX B: EMACS ONE BY ONE

We here provide an overview of the data available for all eMACS clusters.

Specifically, we show colour images of the cluster core generated from HST data (where observations were performed in at least two passbands). Contours (in light grey) of the adaptively smoothed X-ray emission are overlaid for all eMACS clusters observed with CXO to date (see also Fig. 11), while the critical lines for strong gravitational lensing of a source at $z=2$ are shown in yellow for systems for which a lens model is available (see Table 3). We mark multiple-image systems that are used to constrain these lens models and highlight strong-lensing features and (occasionally) cluster galaxies of special interest in insets placed around the main image.

We also show the redshift histogram in the vicinity of each cluster’s systemic redshift and the velocity dispersion derived from it. The spatial distribution of all galaxies with spectroscopic redshifts measured in each field is also presented, where larger symbols mark cluster members (defined as featuring redshifts that fall within $3\sigma$ of the systemic cluster redshift in the radial-velocity histogram); in these plots, the area covered by the shown HST data is outlined by an orange polygon. In both the radial-velocity histograms and the maps showing the location of galaxies with spectroscopic redshifts we highlight in red the BCG (heavy line) and the second-brightest cluster galaxy (light line).

Finally, we list all galaxies with spectroscopic redshifts (whether they are cluster members or not) and mark the BCG (B), any X-ray point sources detected in CXO follow-up observations (X), and candidates for ram-pressure stripping (R). For the remainder of the sample, i.e., for all eMACS clusters that are listed in Table 1 but are not highlighted elsewhere in this paper, the same data summary is provided in an online-only extension of this appendix.

The uncertainty of the tabulated spectroscopic redshifts is typically 0.0005.

Coordinates (J2000) and spectroscopic redshifts of galaxies in the field of eMACSJ0014.9-0056.

<table><thead><tr><th>R.A. (deg)</th><th>Dec (deg)</th><th>z</th></tr></thead><tbody><tr><td>3.6932</td><td>-0.9852</td><td>0.6368</td></tr><tr><td>3.6962</td><td>-0.9689</td><td>0.5351</td></tr><tr><td>3.7025</td><td>-0.9648</td><td>0.5283</td></tr><tr><td>3.7104</td><td>-0.9691</td><td>0.4674</td></tr><tr><td>3.7125</td><td>-0.9655</td><td>0.5276</td></tr><tr><td>3.7148</td><td>-0.9625</td><td>0.5340</td></tr><tr><td>3.7154</td><td>-0.9440</td><td>0.2769</td></tr><tr><td>3.7159</td><td>-0.9484</td><td>0.5278</td></tr><tr><td>3.7181</td><td>-0.9557</td><td>0.5349</td></tr><tr><td>3.7218</td><td>-0.9442</td><td>0.5358</td></tr><tr><td>3.7246</td><td>-0.9387</td><td>0.5321</td></tr><tr><td>3.7253</td><td>-0.9525</td><td>0.5345</td></tr><tr><td>3.7292</td><td>-0.9418</td><td>0.5372</td></tr><tr><td>3.7369</td><td>-0.9367</td><td>0.5404</td></tr><tr><td>3.7428</td><td>-0.9675</td><td>0.5293</td></tr><tr><td>3.7448</td><td>-0.9373</td><td>0.5320</td></tr><tr><td>3.7481</td><td>-0.9216</td><td>0.5707</td></tr><tr><td>3.7511</td><td>-0.9496</td><td>0.5373</td></tr><tr><td>3.7518</td><td>-0.9174</td><td>0.5278</td></tr><tr><td>3.7523</td><td>-0.9370</td><td>0.5302</td></tr><tr><td>3.7553</td><td>-0.9079</td><td>0.5330</td></tr><tr><td>3.7557</td><td>-0.9134</td><td>0.5680</td></tr><tr><td>3.7595</td><td>-0.9084</td><td>0.3697</td></tr><tr><td>3.7696</td><td>-0.9556</td><td>0.5363</td></tr></tbody></table>
<figure><img src="image_26.png" /></figure>

**Figure B1.** eMACSJ0014.9-0056 is an exceptionally X-ray luminous cluster (see Table 4) with a largely relaxed X-ray morphology. The modest number of radial velocities measured for cluster members to date precludes a more robust determination of the cluster velocity dispersion and an assessment of line-of-sight substructure. While strong-lensing features are clearly visible in the cluster core, their faintness makes follow-up spectroscopy challenging; as a result, we currently have no lens model for this system.

This paper has been typeset from a TeX/LATEX file prepared by the author.