# The Next Generation Fornax Survey (NGFS). III. Revealing the Spatial Substructure of the Dwarf Galaxy Population Inside Half of Fornax's Virial Radius

Yasna Ordenes-Briceño, Paul Eigenthaler, Matthew Taylor, Thomas Puzia, Karla Alamo-Martínez, Karen Ribbeck, Roberto P. Muñoz, Hongxin Zhang, Eva Grebel, Simón Ángel, et al.

► To cite this version:

Yasna Ordenes-Briceño, Paul Eigenthaler, Matthew Taylor, Thomas Puzia, Karla Alamo-Martínez, et al.. The Next Generation Fornax Survey (NGFS). III. Revealing the Spatial Substructure of the Dwarf Galaxy Population Inside Half of Fornax's Virial Radius. The Astrophysical Journal, 2018, 859 (1), pp.52. 10.3847/1538-4357/aaba70. hal-03152070

HAL Id: hal-03152070

https://hal.science/hal-03152070v1

Submitted on 17 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# The Next Generation Fornax Survey (NGFS). III. Revealing the Spatial Substructure of the Dwarf Galaxy Population Inside Half of Fornax's Virial Radius

Yasna Ordenes-Briceño<sup>1,2,11</sup> ID, Paul Eigenthaler<sup>1,3,12</sup> ID, Matthew A. Taylor<sup>4,13</sup> ID, Thomas H. Puzia<sup>1</sup> ID,

Karla Alamo-Martínez<sup>1,14</sup> ID, Karen X. Ribbeck<sup>1</sup> ID, Roberto P. Muñoz<sup>1</sup> ID, Hongxin Zhang<sup>1,3,14</sup> ID, Eva K. Grebel<sup>2</sup> ID,

Simón Ángel<sup>1</sup> ID, Patrick Côté<sup>5</sup> ID, Laura Ferrarese<sup>5</sup> ID, Michael Hilker<sup>6</sup> ID, Ariane Lançon<sup>7</sup> ID, Steffen Mieske<sup>8</sup> ID,

Bryan W. Miller<sup>9</sup> ID, Yu Rong<sup>1,3,12</sup> ID, and Ruben Sánchez-Janssen<sup>10</sup> ID

Received 2018 February 7; revised 2018 March 12; accepted 2018 March 26; published 2018 May 23

## Abstract

We report the discovery of 271 previously undetected dwarf galaxies in the outer Fornax cluster regions at radii $r_{vir}/4 < r < r_{vir}/2$ using data from the *Next Generation Fornax Survey* (NGFS) with deep coadded $u'$, $g'$, and $i'$ images obtained with Blanco/DECam at Cerro Tololo Interamerican Observatory. From the 271 dwarf candidates, we find 39 to be nucleated. Together with our previous study of the central Fornax region, the new dwarfs detected with NGFS data number 392, of which 56 are nucleated. The total Fornax dwarf galaxy population from NGFS and other catalogs rises, therefore, to a total of 643 with 181 being nucleated, yielding an overall nucleation fraction of 28%. The absolute $i'$-band magnitudes for the outer NGFS dwarfs are in the range $-18.80 \le M_{i'} \le -8.78$ with effective radii $r_{eff,i'} = 0.18-2.22$ kpc and an average Sérsic index $\langle n \rangle_{i'} = 0.81$. Nonnucleated dwarfs are found to be fainter and smaller by $\Delta\langle M_{i'} \rangle = 2.25$ mag and $\Delta\langle r_{eff,i'} \rangle = 0.4$ kpc than the nucleated dwarfs. We demonstrate a significant clustering of dwarf galaxies on scales $\lesssim 100$ kpc, and projected surface number density profile estimates, $\Sigma_N(r)$, show a concentration of dwarfs in the Fornax core region within $r \lesssim 350$ kpc. $\Sigma_N(r)$ has a flat distribution up to $\sim 350$ kpc, beyond which it declines for the nonnucleated dwarfs. The nucleated dwarfs have a steeper $\Sigma_N(r)$ distribution, are more concentrated toward NGC 1399, and are decreasing rapidly outwards. This is the first time the transition from cluster to field environment has been established for the very faint dwarf galaxy population with robust sample statistics.

**Key words:** galaxies: clusters: individual (Fornax) – galaxies: dwarf – galaxies: elliptical and lenticular, cD

**Supporting material:** machine-readable table

## 1. Introduction

Large numbers of faint, low surface brightness dwarf galaxies are rapidly being discovered in different environments throughout the local universe (e.g., van Dokkum et al. 2015; Muñoz et al. 2015; Müller et al. 2017; Wittmann et al. 2017). The rise of large detector arrays in present-day observatories such as the Dark Energy Camera (DECam; Flaugher et al. 2015) enables us to survey large areas of the sky down to ultra low surface brightness levels, providing the exciting opportunity to search for undiscovered faint dwarf galaxies in massive clusters, galaxy groups, and in the field (e.g., Muñoz et al. 2015; Ferrarese et al. 2016; Müller et al. 2017; Venhola et al. 2017). Dwarf galaxies are found in galaxy group and cluster environments, which are numerically dominated by early-type dwarf galaxies with characteristically

smooth morphologies, exponential surface brightness profiles, and stellar populations consistent with red-sequence galaxies (Sandage & Binggeli 1984; Binggeli et al. 1985; Conselice et al. 2003; Misgeld et al. 2008; den Brok et al. 2011; Ordenes-Briceño et al. 2016; Roediger et al. 2017; Eigenthaler et al. 2018). Early-type dwarf galaxies have typically been classified as dwarf ellipticals (dE), but are also known as dwarf spheroidals (dSph) at fainter magnitudes (Grebel et al. 2003). They exhibit absolute B-band magnitudes fainter than $M_B \simeq -16$, corresponding to $\log(M_*/M_\odot) \lesssim 9$, and effective radii smaller than $\sim 1$ kpc (cf. Figure 8 in Eigenthaler et al. 2018). A further morphological distinction among dwarf galaxies is whether or not they host a central nuclear star cluster. Recent findings show that dwarf nucleation probability is strongly dependent on its spheroid luminosity (Muñoz et al. 2015; Ordenes-Briceño et al. 2018), with the fraction of nucleated dwarfs systematically increasing toward brighter magnitudes.

Large populations of low-mass dwarf galaxies are ideal for studying the dependence of galaxy formation and evolution processes in the transition zones between field and cluster

<footnote n="1"><sup>1</sup> Institute of Astrophysics, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile; yordenes@astro.puc.cl</footnote>

<footnote n="2"><sup>2</sup> Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, D-69120 Heidelberg, Germany</footnote>

<footnote n="3"><sup>3</sup> Chinese Academy of Sciences South America Center for Astronomy and China-Chile Joint Center for Astronomy, Camino El Observatorio 1515, Las Condes, Santiago, Chile</footnote>

<footnote n="4"><sup>4</sup> Gemini Observatory, Northern Operations Center, 670 North A'ohoku Place, Hilo, HI 96720, USA</footnote>

<footnote n="5"><sup>5</sup> NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada</footnote>

<footnote n="6"><sup>6</sup> European Southern Observatory, Karl-Schwarzchild-Str. 2, D-85748 Garching, Germany</footnote>

<footnote n="7"><sup>7</sup> Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l'Université, F-67000 Strasbourg, France</footnote>

<footnote n="8"><sup>8</sup> European Southern Observatory, 3107 Alonso de Córdova, Vitacura, Santiago, Chile</footnote>

<footnote n="9"><sup>9</sup> Gemini Observatory, South Operations Center, Casilla 603, La Serena, Chile</footnote>

<footnote n="10"><sup>10</sup> STFC UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK</footnote>

<footnote n="11"><sup>11</sup> PUC-HD Graduate Student Exchange Fellow.</footnote>

<footnote n="12"><sup>12</sup> CASSACA Postdoctoral Fellow.</footnote>

<footnote n="13"><sup>13</sup> Gemini Science Fellow.</footnote>

<footnote n="14"><sup>14</sup> FONDECYT Postdoctoral Fellow.</footnote>
environments, especially in rich galaxy clusters. Statistically significant samples allow us to study their clustering properties on large scales (Muñoz et al. 2015) and potentially probe the dark matter (DM) fine-structure within the cluster halo. This distribution in return serves as an ideal laboratory for comparisons with predictions from structure formation models (e.g., Bovill et al. 2016).

Recently, observations have revealed that low-mass dwarf galaxies appear to form surprisingly thin planes in the local universe (Pawlowski et al. 2012; Ibata et al. 2013; Tully et al. 2015). The observations of such planes challenge current ΛCDM models of hierarchical structure formation (Kroupa et al. 2005; Pawlowski et al. 2014). The frequency of the occurrence of such anisotropic distributions in dense environments will inform an updated view of structure formation in modern galaxy formation models. These findings show that deep homogeneous surveys are necessary to highlight the spatial distribution of the faint dwarf galaxies in nearby groups and clusters of galaxies without completeness.

In the present work, we attempt to lay the foundation for addressing these issues by investigating the faint dwarf galaxy population in the Fornax galaxy cluster out to half of its virial radius ($r_{vir}$), using data obtained as part of the *Next Generation Fornax Survey* (NGFS). The most prominent survey covering the galaxy cluster outskirts (Ferguson 1989), while seminal, is showing signs of age and suffers from shallow detection limits ($m_B \lesssim 20$ mag for point sources and $\mu_B \lesssim 24$ mag arcsec$^{-2}$ in surface-brightness sensitivity) compared to the potential of modern instrumentation. Given the faint surface brightness of dwarf galaxies recently detected in various galaxy aggregates, the goal of this paper is to update the known population of Fornax dwarf galaxies out to $\lesssim r_{vir}/2$, where one might expect to witness the transition from the central galaxy population to those residing in the cluster outskirts. The Fornax cluster is the nearest high-density region in the southern hemisphere ($m-M = 31.51$ mag or $D_L = 20.0$ Mpc, Blakeslee et al. 2009). Given that its proximity is twice the central galaxy density and its early-type galaxy (ETG) fraction is larger than its northern hemisphere counterpart, the Virgo Cluster, Fornax is an important nearby laboratory to investigate the dependence of galaxy evolution on the dynamical state of the environment.

## 2. Observations and Image Processing

The data presented in this paper is part of the observed NGFS (Muñoz et al. 2015), an ongoing, panchromatic $\sim 30$ deg$^2$ survey of the Fornax galaxy cluster using the *Dark Energy Camera* (DECam; Flaugher et al. 2015) mounted on the 4 m Blanco telescope at Cerro Tololo Interamerican Observatory (CTIO). Figure 1 illustrates the Fornax cluster region that is covered by our inner NGFS footprint, which consists of seven tiles centered on the dominant NGC 1399 galaxy, and homogeneously maps the cluster out to $\sim 50\%$ of its virial radius ($r_{vir} \simeq 1.4$ Mpc; Drinkwater et al. 2001). The dwarf galaxy population in the central tile corresponding to $r \le r_{vir}/4$ was already studied in Muñoz et al. (2015) and Eigenthaler et al. (2018). All tiles are observed in three optical bands reaching point-source detections with $S/N \gtrsim 5$ at 26.5, 26.1, and 25.3 AB mag in the $u'$-, $g'$-, and $i'$-band, respectively.

Initial image processing is carried out by the CTIO Community Pipeline (CP; Valdes et al. 2014), focusing mainly on instrumental signature removal (e.g., bias subtraction,

flat-fielding, cross-talk correction). After the CP-processing, we apply further processing using the ASTROMATIC<fnref n="15" /> software suite to astrometrically calibrate and stack the individual frames and to conduct basic photometry (SCAMP, SWARP, SOURCE EXTRACTOR, hereafter SE; Bertin & Arnouts 1996; Bertin et al. 2002; Bertin 2006). Astrometric and photometric calibration has been performed using the 2MASS astrometric Point Source Catalog (Skrutskie et al. 2006) and SDSS $u'$g'$i'$ stripe 82 standard stars, respectively. To ensure accurate photometric calibration, we cross-verified it with the globular cluster catalog from Kim et al. (2013) in the Fornax area, which was compiled using U-, B-, V-, and I-band photometry taken with MOSAIC-II camera on CTIO/Blanco. We use the empirical transformation equations from Jordi et al. (2006) for the comparison, finding good agreement within the uncertainties. Finally, we proceed to the final image stacking and source detection.

In previous contributions (Muñoz et al. 2015; Eigenthaler et al. 2018), we presented the NGFS results, focusing on the inner $\sim 3$ deg$^2$ ($\lesssim r_{vir}/4$) region of the Fornax cluster. We presented optical colors and structural parameters for $\gtrsim 250$ dwarf galaxy candidates, reaching out to a projected distance of $\sim 350$ kpc from NGC 1399; its spatial distribution suggested a rich and substructured dwarf galaxy population extending well beyond these limits.

In this paper, we expand upon these initial results by using the surrounding DECam pointings (NGFS tiles 2–7; see Figure 1) to identify 271 new dwarf galaxy candidates, of which 39 are nucleated. In the following, we refer to the surrounding NGFS tiles (2–7) as the outer footprint. Together with the 121 previously found NGFS dwarfs from the central tile (Muñoz et al. 2015; Eigenthaler et al. 2018), there is a total of 392 new dwarfs, of which 56 are nucleated, that were discovered in the NGFS data. Our by-eye dwarf detection strategy used for the central tile is again utilized, where several members of the NGFS team (KAM, KXR, MAT, PE, THP, YOB) independently compiled dwarf candidate lists for tiles 2–7 using RGB full-color image stacks, constructed from the individual $u'$g'$i'$ frames. By using cross-matching lists and setting a minimum threshold of three independent detections, we yielded a new robust list of dwarf galaxy candidates projected within $r_{vir}/2$ of NGC 1399. While all frames are fully reduced, we defer a full-color and stellar population analysis to a future paper and limit the scope of the present work to a monochromatic $i'$-band presentation of magnitudes, structural parameters, and spatial distribution characteristics.

We complement our final catalog with the known dwarf galaxy population in Fornax, using the likely members in the Fornax Cluster Catalog (FCC; Ferguson 1989). In the outer NGFS footprint ($0.25 < R/R_{vir} \le 0.5$), we find a total of 114 FCC galaxies (29.6%) and in the so-far searched NGFS survey area, a total of 251 literature galaxies (39%; Ferguson 1989; Mieske et al. 2007). Taking into account the dwarfs from existing catalogs and the new NGFS dwarfs, the total Fornax dwarf galaxy population reported in this work consists of 643 dwarfs of which 462 are nonnucleated and 181 are nucleated.

## 3. Structural Parameters of the Dwarf Candidates

The surface brightness profiles for the dwarfs are studied with GALFIT (v3.0.5 Peng et al. 2002) using a S´ersic profile (S´ersic 1963; Caon et al. 1993). Our procedure has been

<footnote n="15">¹⁵ http://www.astromatic.net/software</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Illustration of the spatial distribution of Fornax dwarf galaxy candidates in a grayscale underlying image from DSS. The footprint of the NGFS survey (inside ~ r<sub>vir</sub>/2), which is indicated by the gray shaded DECam tiles with tile 1 for the central region and tiles 2–7 for the outer region (see also NGFS footprint in Figure 1 Eigenthaler et al. 2018), labeled with their respective numbers (middle panel). The positions of nonnucleated dwarfs are shown by red open circles while the nucleated dwarfs are marked by red circles with a central dot. The left and right panels show examples of the newly detected dwarfs in the different NGFS footprint tiles (2–7). Each stamp image contains the NGFS tile number (bottom right), which contains the corresponding dwarfs galaxy as well as the scale bar (solid line) showing 10''/3 ≃ 1 kpc at the bottom left of each panel. The gray-dashed circles show NGC 1399-centric distances of r<sub>vir</sub>/4 (≈350 kpc) and r<sub>vir</sub>/2 (≈700 kpc).</figcaption></figure>

described in detail in Muñoz et al. (2015) and Eigenthaler et al. (2018). It is an iterative process where the light profile is approximated with a one-component fit to the two-dimensional (2D) galaxy surface brightness distribution. We run GALFIT on cutout images of 105" × 105" in size (≈10.2 kpc × 10.2 kpc) using object masks created from SE segmentation maps and PSF models created with PSFEX (Bertin 2011). For nucleated dwarfs, we iterate the method described above several times and improve the object mask in each iteration step until the nucleus is completely masked (see Eigenthaler et al. 2018; Ordenes-Briceño et al. 2018, for more details). The final fitting profile considers only the spheroid light component of the dwarf galaxy, leaving a residual image (original-spheroid model) with the nuclear cluster in the galaxy center. The analysis of the nuclear star clusters will be presented in an upcoming NGFS contribution. In the following, we focus on the structural parameters of the spheroid sample. The dwarf candidates from the outer footprint have absolute i'-band magnitudes in the range -18.80 ≤ Mᵢ' ≤ -8.78 with photometric errors <0.1 mag, effective radii between 1''/8 and 22''/8 (r<sub>eff,i'</sub> = 0.18–2.22 kpc at the Fornax cluster distance of D<sub>L</sub> = 20.0 Mpc), a mean Sérsic index of ⟨n⟩ᵢ = 0.81, and an average axis ratio of ⟨b/a⟩ᵢ' = 0.69.

Comparing the central and outer regions in terms of mean structural parameters and nucleation fraction, we see some interesting differences mostly in the mean magnitudes and effective radii. Nonnucleated dwarfs in the central region are brighter and larger with ⟨Mᵢ'⟩ = -11.99 ± 0.12 mag and ⟨r<sub>eff,i'</sub⟩ = 0.61 ± 0.03 kpc, relative to nonnucleated dwarfs in the outer region, which have ⟨Mᵢ'⟩ = -11.65 ± 0.10 mag and ⟨r<sub>eff,i'</sub⟩ = 0.55 ± 0.02 kpc. Nucleated dwarfs in the central region have significantly fainter average luminosities but are similar in average size compared to their nucleated counterparts in the outer footprint, ⟨Mᵢ'⟩ = -12.43 ± 0.21 mag, ⟨r<sub>eff,i'</sub⟩ = 0.91 ± 0.04 kpc (central) and ⟨Mᵢ'⟩ = -13.87 ± 0.2 mag and ⟨r<sub>eff,i'</sub⟩ = 0.95 ± 0.04 kpc (outer). These differences are more pronounced when comparing the nucleated and nonnucleated dwarf population, i.e., the nonnucleated dwarf population is fainter than nucleated dwarfs by Δ⟨Mᵢ'⟩ = 0.44 mag in the central region and Δ⟨Mᵢ'⟩ = 2.25 mag in the outer region. In addition, the nonnucleated dwarf population is, on average, smaller than the nucleated dwarf population by Δ⟨r<sub>eff,i'</sub⟩ = 0.3 kpc and Δ⟨r<sub>eff,i'</sub⟩ = 0.4 kpc, in the central and outer regions, respectively.

Nonnucleated dwarfs have similar mean sizes and luminosities independent of local environmental density, i.e., central
<table><caption>**Table 1**
Dwarf Galaxies in the Fornax Cluster</caption><thead><tr><th>ID</th><th>α<sub>2000</sub></th><th>δ<sub>2000</sub></th><th>m<sub>i</sub><br>(mag)</th><th>M<sub>i</sub><sup>a</sup><br>(mag)</th><th>n<sup>b</sup></th><th>r<sub>eff</sub><br>(arcsec)</th><th>r<sub>eff</sub><sup>a</sup><br>(kpc)</th><th>Type<sup>c</sup></th><th>Reference</th></tr></thead><tbody><tr><td>NGFS033260-341909</td><td>03:32:59.92</td><td>-34:19:08.8</td><td>20.07</td><td>-11.44</td><td>1.3208</td><td>5.13</td><td>0.497</td><td>○</td><td></td></tr><tr><td>NGFS033304-334329</td><td>03:33:03.61</td><td>-33:43:29.2</td><td>15.78</td><td>-15.73</td><td>1.2081</td><td>12.90</td><td>1.251</td><td>⊙</td><td>FCC111</td></tr></tbody></table>

**Notes.**

<sup>a</sup> Assuming a distance modulus of $(m - M)_0 = 31.51$ mag or $D_L = 20.0$ Mpc (Blakeslee et al. 2009).

<sup>b</sup> Sérsic index (Sérsic 1963; Caon et al. 1993).

<sup>c</sup> Morphological galaxy type classification: ⊙ = nucleated, ○ = nonnucleated dwarf galaxy.

(This table is available in its entirety in machine-readable form.)

versus outer region. However, central nucleated dwarfs are on average about 1.4 mag fainter than nucleated dwarfs in the outer regions. In the central regions, nucleated and nonnucleated have similar magnitudes but different mean sizes. However, in the outer region, nucleated dwarfs are on average brighter and larger than the nonnucleated dwarfs. Table 1 lists the IDs, coordinates, and structural parameters for the dwarf candidates at cluster-centric radii $r_{NGC 1399} > r_{vir}/4$, complementing the sample from Eigenthaler et al. (2018) for the inner region ($r_{NGC 1399} < r_{vir}/4$).

# 4. Discussion

## 4.1. Size-Luminosity Relation

Scaling relations are useful tools to gain insight into the link of the formation processes between different astronomical objects. We illustrate in Figure 2 the size-luminosity relation in terms of the effective radius and absolute $i'$-band magnitude for the entire NGFS dwarf sample. A total of 452 nonnucleated and 178 nucleated dwarfs are shown. Thirteen dwarfs do not have structural parameter information due to very low surface brightness and/or complicated contamination in their nearby environment (e.g., bright star spikes, detector blemishes, crowded field). To map a large luminosity range, we overplot different galaxy samples, including Local Group dwarfs (McConnachie 2012), ultra-diffuse galaxies from the Coma and Virgo clusters (Mihos et al. 2015; van Dokkum et al. 2015), and giant ellipticals from Fornax, Virgo and the Carnegie-Irvine catalog (Ferrarese et al. 2006; Ho et al. 2011). The NGFS dwarf sample by itself covers a range in absolute magnitude of $-18.80 \le M'_{i'} \le -8.78$ mag and effective radii $r_{eff,i'} = 0.11-2.72$ kpc, comprising an effective surface brightness from $\langle\mu_{i'}\rangle_e = 20-28$ mag arcsec$^{-2}$. The sequence of giant ellipticals stretching from the upper-to-center-right of the diagram is connected to that of the NGFS dwarfs and dSphs (center to lower-left) by an intermediate bridge of galaxies. The bridge spans the $-20 < M'_{i'} < -15$ mag, $0.6 \le r_{eff,i'}/kpc \le 2$ parameter space and blends the brightest NGFS dwarfs with the faint regime of ETGs. We note here that the dwarfs in this group consist primarily of nucleated candidates such that 44/63 of these dwarfs show clear nuclei.

UDGs seem to follow their own sequence, roughly along constant effective surface brightness, avoiding the bridge between dwarf and giant galaxies. Although UDGs seem to have similar magnitudes to the brightest dwarf galaxies, they are much more extended. UDGs have been detected in multiple environments (e.g., Koda et al. 2015; Mihos et al. 2015; Muñoz et al. 2015; van Dokkum et al. 2015; Janssens et al. 2017; Lee et al. 2017; Venhola et al. 2017). They show signatures of

massive DM halos (e.g., Beasley et al. 2016; van Dokkum et al. 2016), and their population size scales with the mass of the central halo (van der Burg et al. 2016, 2017; Janssens et al. 2017). Together with predictions from theoretical studies, this suggests that UDGs are a consequence of rapidly spinning, massive halos ($\gtrsim 10^{10} M_{\odot}$) that recently fell into denser environments (e.g., Rong et al. 2017; Amorisco 2018). In our NGFS footprint, six UDG candidates are found in the central region and one in the outer region. Their magnitudes and effective radii are in the range $-15.62 \le M'_{i'} \le -13.85$ mag and $r_{eff,i'} = 1.79 - 2.72$ kpc, respectively. Of the seven candidates, two harbor a nuclear star cluster. Their properties make them very similar to UDGs found in other galaxy cluster environments, such as Coma and Virgo (Mihos et al. 2015; van Dokkum et al. 2015). A curious Local Group counterpart is the currently disrupting Sagittarius dwarf galaxy ($M_V = -13.5$ mag and $r_{eff} = 2.6$ kpc, McConnachie 2012) with its nucleus and the central star cluster M54 (Bellazzini et al. 2008; Mucciarelli et al. 2017).

Figure 2 shows that the nucleation fraction of the NGFS dwarf sample decreases strongly with luminosity. The overall nucleation fraction is 28% for the entire luminosity range of the NGFS dwarfs. Nonetheless, the nucleation fraction reaches ~90% at bright magnitudes, i.e., $M'_{i'} \lesssim -17$ mag and drops to zero at the faintest galaxy luminosities, i.e., $M'_{i'} \gtrsim -9.56$ mag. This limit marks the currently faintest nucleated galaxy in the NGFS dwarf galaxy sample (see also Muñoz et al. 2015; Ordenes-Briceño et al. 2018).

## 4.2. Spatial Distribution

Figure 3 shows the spatial distribution of the dwarf galaxy candidates in the NGFS survey region, with dashed black circles indicating NGC 1399-centric radii of $r_{vir}/4$ and $r_{vir}/2$ that correspond to ~350 and 700 kpc at the distance of Fornax. The dwarfs and giant galaxies are distributed throughout the field with the projected dwarf galaxy surface number density profile, $\Sigma_N(\alpha, \delta)$, shown by the color shading, computed along a $15 \times 15$ bins grid—corresponding to physical bin sizes of $152 \times 146$ kpc$^2$—and show the resulting 2D histogram smoothed with Lanczos interpolation. We also estimate $\Sigma_N(\alpha, \delta)$ by a nonparametric kernel density estimate using a Gaussian kernel of 0.25 bandwidth and show resulting curves of iso-density contours by white-scaled thin solid lines. We point out that variations in surface brightness limits due to bright galaxy haloes do not affect the results, as the typical size of a galaxy halo is negligible compared to the area studied here and the structures found therein.

Both $\Sigma_N(\alpha, \delta)$ estimates show a general concentration of dwarfs in the core regions of Fornax within
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Size-luminosity relation for dwarf and giant galaxies in Fornax and the nearby universe (see the legend at the bottom right). Galaxy size is given by the effective radius and the luminosity represented as the absolute $i'$-band magnitude. The NGFS dwarf population is shown with red circles containing black dots, indicating the nucleated galaxies. The data for bright Fornax ETGs were taken from the HyperLeda database (http://leda.univ-lyon1.fr), where the effective radii were computed, in order of preference, from Caon et al. (1994), de Vaucouleurs et al. (1991), and Lauberts & Valentijn (1989). Lines of constant average effective surface brightness are indicated for $\langle\mu_{i'}\rangle_e = 28, 26, 24, 22$ mag arcsec$^{-2}$. An approximate surface brightness limit of our NGFS data is illustrated by the shaded region toward the top left. The spatial resolution limit ($0''/8$ in the $i'$ filter, corresponding to $\sim 78$ pc) is indicated by the lightly shaded region at the bottom part of the diagram; below this limit, objects appear as unresolved point sources in our NGFS data at the distance of Fornax ($m - M = 31.51$ mag or $D_L = 20.0$ Mpc, Blakeslee et al. 2009).</figcaption></figure>

$r \lesssim 350$ kpc. NGC 1399 itself can be seen to occupy an apparent saddle-point between two main dwarf galaxy overdensities toward the East and West (see Figures 1 and 3, and also Muñoz et al. 2015). The projected distribution of dwarfs in the western over-density generally follows that of the giant galaxies, which may suggest a physical association. While the current data cannot confirm such a connection, we note the contrast with the $\sim 200$ kpc scale over-density to the east. This group shows a more regular morphology and lies in between to the projected positions of only two bright galaxies, complicating the notion of physical origins with nearby giant hosts. Weaker density contrasts are found for a third “orphan” group of dwarf candidates located near $(\alpha, \delta) \approx (03^h33^m, -35^\circ75)$.

The top two left-hand panels of Figure 4 show the $\Sigma_N(\alpha, \delta)$ profile calculated with respect to NGC 1399. To guard against potential biases introduced in choosing arbitrary bin sizes, we resample the dwarf galaxies using both a constant and an adaptive binning strategy. For the former, we choose constant bins between 5' and 25', in steps of 1'. At each step, we calculate $\Sigma_N(\alpha, \delta)$ in each of the corresponding annuli and show the aggregated data as dots. We have done this procedure for nonnucleated and nucleated dwarfs and the entire dwarf sample. Similarly, the middle panel of Figure 4 shows the data

based on adaptive bin sizes with the three samples described above. Here, bins are chosen such that each of them contains exactly $N$ dwarf candidates with $5 \le N \le 25$. The constant binning produces a much smoother variation in $\Sigma_N(\alpha, \delta)$, while the adaptive binning shows a sensitivity to local dwarf overdensities resulting in artificially small annuli and corresponding spikes in the $\Sigma_N(\alpha, \delta)$ profile. We approximate the dwarf $\Sigma_N(\alpha, \delta)$ profiles by a power-law model, following other radially dependent projected densities (e.g., $\Sigma_N(\alpha, \delta) \propto \exp(A R^\alpha$); Sérsic 1963; Einasto 1965) by Markov-Chain Monte Carlo (MCMC) sampling. We use MCMC with a normal prior to sample the posterior probability of $(A, \alpha) = 10^5$ times, which allows sufficient burn-in to skip over stochastic or unreliable results from early iterations so that each chain converges to consistent, well-defined peaks in the marginalized parameter estimate distributions. The resulting $\Sigma_N(r)$ estimates are shown in the two top left panels of Figure 4 as solid curves. The text displays the means of the posterior probability density functions for $\alpha$ alongside the corresponding 95% confidence limits. The projected radial surface number density for the nonnucleated dwarfs has a flat distribution up to $\sim 350$ kpc and slowly declines beyond that radial distance. On the other hand, nucleated dwarfs have a steeper $\Sigma_N(r)$ distribution than nonnucleated dwarfs, being more concentrated
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Distribution of dwarf galaxies (fainter than $M_{f'} \approx -19$ mag) in the Fornax galaxy cluster within about half of its virial radius ($\sim r_{vir}/2$) shown as a surface number density distribution (see colorbar scale). Overplotted are the NGFS dwarf candidates (red dots) and bright Fornax galaxies ($M_{f'} \lesssim -19$ mag, orange squares) with NGC 1399 shown by the large square. The black dashed circles show NGC 1399-centric distances of $r_{vir}/4$ ($\sim 350$ kpc) and $r_{vir}/2$ ($\sim 700$ kpc), while the solid contours represent 2D Gaussian KDEs with a 0.25 kernel width.</figcaption></figure>

in the inner regions near the cD galaxy NGC 1399 and decreasing rapidly outwards. This is similar to the results of Lisker et al. (2007) who found, based on SDSS photometry ($M_B \lesssim -13$ mag), that the bright nucleated dwarf galaxy population in the Virgo galaxy cluster is more centrally concentrated than the nonnucleated dwarf population.

The bottom left-panel of Figure 4 shows the results of a two-point angular correlation function ($\hat{w}(\theta)$; Landy & Szalay 1993) analysis of the dwarf galaxy candidates, using the same three samples (nonnucleated, nucleated, and all dwarfs). The two-point angular correlation function quantifies the excess probability of finding galaxy pairs at a given angular separation over a random distribution without a restriction to Gaussianity (e.g., Connolly et al. 2002; Sato et al. 2009) and is typically used to constrain cosmological parameters and structure formation models (e.g., Bernardeau et al. 2002; Cooray & Sheth 2002; Tegmark et al. 2002, 2004; Dolney et al. 2006). Noting the steepening of the $\Sigma_N(\alpha, \delta)$ profile at $\sim 350$ kpc and the $\Sigma_N(r)$ distribution, we split the total sample into those dwarf galaxies inside and outside of this radius and estimate $\hat{w}(\theta)$ for each subpopulation. The solid lines along with $\pm 1\sigma$ bounds show the results for the three samples, in addition to all outer and all inner

dwarfs, with correlation lengths $\theta$ binned in steps of $10'$ for the total, nonnucleated, and nucleated samples, and $5'$ for the two subsamples. Estimating $\hat{w}(\theta)$ using different bin sizes reveals mild deviations but with the overall behaviors unchanged.

Given that the likelihood of finding two points separated by an angular distance $\theta$ compared to a purely uniform distribution is encapsulated by $\hat{w}(\theta)$, we find particularly strong evidence for dwarf clustering on sub-100 kpc scales for the overall and outer dwarf population. In particular, the outer dwarf galaxies appear more likely to be clustering on scales approaching $\sim 50$ kpc with a notable decrease on scales $\gtrsim 100$ kpc. Overall, the apparent smaller clustering scale appears superimposed on the $\sim 350$ kpc scale over-density shown in Figure 3. This larger clustering scale is reflected by the almost flat $\Sigma_N(\alpha, \delta)$ profile within $\sim 350$ kpc, outside of which a general steepening of the profile is seen, modulo local, projected dwarf clustering and sub-groups. Conversely, the dwarf population inside of $r \approx 350$ kpc does not appear to show evidence as strong for clustering at any scale, but we note that the limited spatial region will tend to mute $\hat{w}(\theta)$. In any case, we check against an underlying uniform distribution of dwarf galaxies by creating a large artificial set of three-dimensional dwarf galaxy
obtained with the Dark Energy Camera (DECam), which was
constructed by the Dark Energy Survey (DES) collaboration.
Facility: CTIO:Blanco/DECam.

Software: astropy (Astropy Collaboration et al. 2013),
matplotlib (Hunter 2007), scikit-learn, (Pedregosa et al. 2012),
astroML, (VanderPlas et al. 2012) SCAMP (Bertin et al. 2002),
SWARP (Bertin 2006), Source Extractor (Bertin & Arnouts
1996), Galfit (Peng et al. 2002).

ORCID iDs

Yasna Ordenes-Briceño https://orcid.org/0000-0001-7966-7606
Paul Eigenthaler https://orcid.org/0000-0001-8654-0101
Matthew A. Taylor https://orcid.org/0000-0003-3009-4928
Thomas H. Puzia https://orcid.org/0000-0003-0350-7061
Karla Alamo-Martínez https://orcid.org/0000-0002-5897-7813
Karen X. Ribbeck https://orcid.org/0000-0002-3004-4317
Roberto P. Muñoz https://orcid.org/0000-0003-1743-0456
Hongxin Zhang https://orcid.org/0000-0003-1632-2541
Eva K. Grebel https://orcid.org/0000-0002-1891-3794
Simón Ángel https://orcid.org/0000-0002-5322-9189
Patrick Côté https://orcid.org/0000-0003-1184-8114
Laura Ferrarese https://orcid.org/0000-0002-8224-1128
Michael Hilker https://orcid.org/0000-0002-2363-5522
Ariane Lançon https://orcid.org/0000-0002-7214-8296
Steffen Mieske https://orcid.org/0000-0003-4197-4621
Bryan W. Miller https://orcid.org/0000-0002-5665-376X
Yu Rong https://orcid.org/0000-0002-2204-6558
Ruben Sánchez-Janssen https://orcid.org/0000-0003-4945-0056

References

Amorisco, N. C. 2018, MNRAS, 475, L116

Annibali, F., Nipoti, C., Ciotti, L., et al. 2016, ApJL, 826, L27

Astropy Collaboration, Robitaille, T. P., & Tollerud, E. J. 2013, A&A,
558, A33

Beasley, M. A., Romanowsky, A. J., Pota, V., et al. 2016, ApJL, 819, L20

Bellazzini, M., Ibata, R. A., Chapman, S. C., et al. 2008, AJ, 136, 1147

Bernardeau, F., Colombi, S., Gaztañaga, E., & Scoccimarro, R. 2002, PhR,
367, 1

Bertin, E. 2006, adass XV, 351, 112

Bertin, E. 2011, adass XX, 442, 435

Bertin, E., & Arnouts, S. 1996, A&AS, 117, 393

Bertin, E., Mellier, Y., Radovich, M., et al. 2002, in ASP Conf. Ser. 281, Astronomical Data Analysis Software and Systems XI, ed. D. A. Bohlender, D. Durand, & T. H. Handley (San Francisco, CA: ASP), 228

Binggeli, B., Sandage, A., & Tammann, G. A. 1985, AJ, 90, 1681

Blakeslee, J. P., Jordán, A., Mei, S., et al. 2009, ApJ, 694, 556

Bovill, M. S., Puzia, T. H., Ricotti, M., & Taylor, M. A. 2016, ApJ, 832, 88

Caon, N., Capaccioli, M., & D'Onofrio, M. 1993, MNRAS, 265, 1013

Caon, N., Capaccioli, M., & D'Onofrio, M. 1994, A&AS, 106, 199

Connolly, A. J., Scranton, R., Johnston, D., et al. 2002, ApJ, 579, 42

Conselice, C. J., Gallagher, J. S., III, & Wyse, R. F. G. 2003, AJ, 125, 66

Cooray, A., & Sheth, R. 2002, PhR, 372, 1

de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H. G., Jr., et al. 1991, in
Third Reference Catalogue of Bright Galaxies, Vol. I: Explanations and
references. Vol. II: Data for galaxies between 0h and 12h. Volume III: Data
for galaxies between 12h and 24h, ed. G. de Vaucouleurs et al. (New York:
Springer), 2091

den Brok, M., Peletier, R. F., Valentijn, E. A., et al. 2011, MNRAS, 414, 3052

Dolney, D., Jain, B., & Takada, M. 2006, MNRAS, 366, 884

Drinkwater, M. J., Gregg, M. D., Holman, B. A., & Brown, M. J. I. 2001,
MNRAS, 326, 1076

Eigenthaler, P., Puzia, T. H., Taylor, M. A., et al. 2018, ApJ, 855, 142

Einasto, J. 1965, TrAlm, 5, 87

Einasto, J., & Haud, U. 1989, A&A, 223, 89

Ferguson, H. C. 1989, AJ, 98, 367

Ferrarese, L., Côté, P., Jordán, A., et al. 2006, ApJS, 164, 334

Ferrarese, L., Côté, P., Sánchez-Janssen, R., et al. 2016, ApJ, 824, 10

Flaugher, B., Diehl, H. T., Honscheid, K., et al. 2015, AJ, 150, 150

Grebel, E. K., Gallagher, J. S., III, & Harbeck, D. 2003, AJ, 125, 1926

Ho, L. C., Li, Z.-Y., Barth, A. J., Seigar, M. S., & Peng, C. Y. 2011, ApJS,
197, 21

Hunter, J. D. 2007, CSE, 9, 90

Ibata, R. A., Lewis, G. F., Conn, A. R., et al. 2013, Natur, 493, 62

Janssens, S., Abraham, R., Brodie, J., et al. 2017, ApJL, 839, L17

Jordi, K., Grebel, E. K., & Ammon, K. 2006, A&A, 460, 339

Kim, H.-S., Yoon, S.-J., Sohn, S. T., et al. 2013, ApJ, 763, 40

Koda, J., Yagi, M., Yamanoi, H., & Komiyama, Y. 2015, ApJL, 807, L2

Kroupa, P., Theis, C., & Boily, C. M. 2005, A&A, 431, 517

Landy, S. D., & Szalay, A. S. 1993, ApJ, 412, 64

Lauberts, A., & Valentijn, E. A. 1989, The Surface Photometry Catalogue of
the ESO-Uppsala Galaxies (Garching: European Southern Observatory)

Lee, M. G., Kang, J., Lee, J. H., & Jang, I. S. 2017, ApJ, 844, 157

Lisker, T., Grebel, E. K., Binggeli, B., & Glatt, K. 2007, ApJ, 660, 1186

Martínez-Delgado, D., Romanowsky, A. J., Gabany, R. J., et al. 2012, ApJL,
748, L24

McConnachie, A. W. 2012, AJ, 144, 4

Mieske, S., Hilker, M., Infante, L., & Mendes de Oliveira, C. 2007, A&A,
463, 503

Mihos, J. C., Durrell, P. R., Ferrarese, L., et al. 2015, ApJL, 809, 21

Misgeld, I., Mieske, S., & Hilker, M. 2008, A&A, 486, 697

Mucciarelli, A., Bellazzini, M., Ibata, R., et al. 2017, A&A, 605, A46

Müller, O., Jerjen, H., & Binggeli, B. 2017, A&A, 597, A7

Muñoz, R. P., Eigenthaler, P., Puzia, T. H., et al. 2015, ApJL, 813, L15

Ordenes-Briceño, Y., Eigenthaler, P., Taylor, M. A., Puzia, T. H., et al. 2018,
ApJ, in press (arXiv:1805.00491)

Ordenes-Briceño, Y., Taylor, M. A., Puzia, T. H., et al. 2016, MNRAS,
463, 1284

Pawlowski, M. S., Famaey, B., Jerjen, H., et al. 2014, MNRAS, 442, 2362

Pawlowski, M. S., Pflamm-Altenburg, J., & Kroupa, P. 2012, MNRAS,
423, 1109

Pedregosa, F., Varoquaux, G., Gramfort, A., et al. 2012, JMLR, 12, 2825

Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H.-W. 2002, AJ, 124, 266

Roediger, J. C., Ferrarese, L., Côté, P., et al. 2017, ApJ, 836, 120

Rong, Y., Guo, Q., Gao, L., et al. 2017, MNRAS, 470, 4231

Sandage, A., & Binggeli, B. 1984, AJ, 89, 919

Sato, M., Hamana, T., Takahashi, R., et al. 2009, ApJ, 701, 945

Sérsic, J. L. 1963, BAAA, 6, 41

Skrutskie, M. F., Cutri, R. M., Stiening, R., et al. 2006, AJ, 131, 1163

Stierwalt, S., Liss, S. E., Johnson, K. E., et al. 2017, NatAs, 1, 0025

Tegmark, M., Blanton, M. R., Strauss, M. A., et al. 2004, ApJ, 606, 702

Tegmark, M., Dodelson, S., Eisenstein, D. J., et al. 2002, ApJ, 571, 191

Tully, R. B., Libeskind, N. I., Karachentsev, I. D., et al. 2015, ApJL, 802,
L25

Valdes, F., Gruendl, R. & DES Project 2014, adass XXIII, 485, 379

van der Burg, R. F. J., Hoekstra, H., Muzzin, A., et al. 2017, A&A, 607, A79

van der Burg, R. F. J., Muzzin, A., & Hoekstra, H. 2016, A&A, 590, A20

van Dokkum, P., Abraham, R., Brodie, J., et al. 2016, ApJL, 828, L6

van Dokkum, P. G., Abraham, R., Merritt, A., et al. 2015, ApJL, 798, L45

VanderPlas, J., Connolly, A. J., Ivezic, Z., & Gray, A. 2012, in Proc. Conf.
Intelligent Data Understanding, 47

Venhola, A., Peletier, R., Laurikainen, E., et al. 2017, A&A, 608, A142

Wetzel, A. R., Deason, A. J., & Garrison-Kimmel, S. 2015, ApJ, 807, 49

Wheeler, C., Oñorbe, J., Bullock, J. S., et al. 2015, MNRAS, 453, 1305

Wittmann, C., Lisker, T., Ambachew Tilahun, L., et al. 2017, MNRAS,
470, 1512