# The Lyman-alpha and Continuum Origins Survey I: Survey description and Lyα imaging

Alexandra Le Reste, Claudia Scarlata, Matthew Hayes, Jens Melinder, Alberto Saldana-Lopez, Axel Runnholm, Yu-Heng Lin, Ricardo O Amorín, Hakim Atek, Sanchayeeta Borthakur, et al.

► To cite this version:

Alexandra Le Reste, Claudia Scarlata, Matthew Hayes, Jens Melinder, Alberto Saldana-Lopez, et al.. The Lyman-alpha and Continuum Origins Survey I: Survey description and Lyα imaging. The Astrophysical Journal Supplement, 2025, 280 (1), pp.27. 10.3847/1538-4365/adf227 . hal-05057656

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# The Lyα and Continuum Origins Survey. I. Survey Description and Lyα Imaging

Alexandra Le Reste¹, Claudia Scarlata¹, Matthew J. Hayes², Jens Melinder², Alberto Saldana-Lopez², Aaron Smith³, Axel Runnholm², Yu-Heng Lin⁴, Ricardo O. Amorín⁵, Hakim Atek⁶, Sanchayeeta Borthakur⁷, Cody A. Carr⁸,⁹, Brian Fleming¹⁰, Sophia R. Flury¹¹, Mauro Giavalisco¹², Alaina Henry¹³,¹⁴, Anne E. Jaskot¹⁵, Zhiyuan Ji¹⁶, Intae Jung¹⁴, Floriane Leclercq¹⁷, Rui Marques-Chaves¹⁸, Stephan R. McCandliss¹⁹, M. S. Oey²⁰, Göran Östlin², Swara Ravindranath²¹,²², Daniel Schaerer²¹,²³, Trinh X. Thuan²⁴, and Xinfeng Xu²⁵,²⁶

¹Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA

²Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden

³Department of Physics, The University of Texas at Dallas, Richardson, TX 75080, USA

⁴Caltech/IPAC, 1200 E. California Boulevard, Pasadena, CA 91125, USA

⁵Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain

⁶Institut d'Astrophysique de Paris, CNRS, Sorbonne Université, 98bis Boulevard Arago, 75014, Paris, France

⁷School of Earth and Space Exploration, Arizona State University, 781 Terrace Mall, Tempe, AZ 85287, USA

⁸Center for Cosmology and Computational Astrophysics, Institute for Advanced Study in Physics, Zhejiang University, Hangzhou 310058, People's Republic of China

⁹Institute of Astronomy, School of Physics, Zhejiang University, Hangzhou 310058, People's Republic of China

¹⁰Laboratory for Atmospheric and Space Physics, University of Colorado, 3665 Discovery Drive, Boulder, CO 80303, USA

¹¹Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK

¹²University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA

¹³Center for Astrophysical Sciences, Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

¹⁴Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

¹⁵Department of Astronomy, Williams College, Williamstown, MA 01267, USA

¹⁶Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA

¹⁷CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, Univ Lyon, Univ Lyon1, Ens de Lyon, F-69230 Saint-Genis-Laval, France

¹⁸Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

¹⁹Johns Hopkins University, Department of Physics & Astronomy, Center for Astrophysical Sciences, 3400 North Charles Street, Baltimore, MD 21218, USA

²⁰Astronomy Department, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA

²¹Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA

²²Center for Research and Exploration in Space Science and Technology II, Department of Physics, Catholic University of America, 620 Michigan Avenue N.E., Washington, DC 20064, USA

²³CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France

²⁴Astronomy Department, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325, USA

²⁵Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA

²⁶Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), 1800 Sherman Avenue, Evanston, IL 60201, USA

Received 2025 April 9; revised 2025 July 3; accepted 2025 July 17; published 2025 September 2

## Abstract

Understanding the mechanisms driving the escape of ionizing or Lyman continuum (LyC) emission from the interstellar medium of galaxies is necessary to constrain the evolution of reionization and the sources responsible for it. While progress has been made in identifying the global galaxy properties linked to the ionizing escape fraction $f_{\text{esc}}^{\text{LyC}}$, little is currently known about how spatially resolved galaxy properties impact it. We present Hubble Space Telescope (HST) imaging data obtained in the Lyα and Continuum Origins Survey (LaCOS). LaCOS consists of HST imaging covering rest-frame optical and UV bands for a subsample of 42 galaxies in the Low redshift Lyman Continuum Survey, including 22 LyC emitters ($f_{\text{esc}}^{\text{LyC}} = 0.01-0.49$). Here, we describe the sample, observations, and data reduction, and investigate connections between global and sub-kiloparsec Lyα emission and $f_{\text{esc}}^{\text{LyC}}$. We confirm the correlation between $f_{\text{esc}}^{\text{LyC}}$ and EW$_{Lyα}$, and the anticorrelation with $r_{50}$, when using values obtained via global photometry. We also find correlations previously found with spectroscopy with global photometric $L_{Lyα}$, $f_{\text{esc}}^{\text{Lyα}}$, $\Sigma_{SFR}$, and $f_{\text{esc}}^{\text{LyC}}$, but with a smaller degree of correlations ($|\Delta\tau| \sim 0.1$). We find correlations are strongest between Lyα observables ($L_{Lyα}$, EW$_{Lyα}$) and $f_{\text{esc}}^{\text{LyC}}$ when measured in a small aperture around the brightest UV source in each galaxy. We interpret these results as evidence that LyC photons escaping on the line of sight are contributed by a small number of UV-bright compact regions in most galaxies in LaCOS.

*Unified Astronomy Thesaurus concepts: Galaxies (573); Emission line galaxies (459); Lyman-alpha galaxies (978); Ultraviolet astronomy (1736); Ultraviolet sources (1741); Reionization (1383)*

## 1. Introduction

The Epoch of Reionization (EoR) is a key cosmological period at $z \ge 5.5$ (R. H. Becker et al. 2001; X. Fan et al. 2006;

S. E. I. Bosman et al. 2022) during which the first sources of light almost completely ionized hydrogen in the intergalactic medium (IGM). The onset and progress of reionization largely depend on the number and ionizing efficiency of sources at this epoch emitting in the Lyman continuum (LyC) range, with $h\nu > 13.6$ eV or $\lambda < 912$ Å. The globally averaged ionization rate density $\dot{n}_{\text{ion}}$ is the product of three parameters: the UV luminosity density function $\rho_{\text{UV}}$, the ionizing photons

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
production efficiency $\xi_{\text{ion}}$, and the escape fraction of ionizing photons $f_{\text{esc}}^{\text{LyC}}$ (e.g., B. E. Robertson 2022). These parameters not only offer insights into the evolution of the last major phase transition in the Universe but also encode important information about the first sources of light and the physics of the interstellar and circumgalactic media in the early Universe. As a result, significant effort has been made in both observational and computational astrophysics in the past decades to constrain these three parameters for different UV-emitting sources.

Observations of galaxies into the EoR with the Hubble Space Telescope (HST) and the James Webb Space Telescope have yielded improved estimations of the UV luminosity function (Y. Harikane et al. 2023; S. L. Finkelstein et al. 2023, 2024; P. G. Pérez-González et al. 2023; N. J. Adams et al. 2024; M. B. Bagley et al. 2024; D. J. McLeod et al. 2024; C. T. Donnan et al. 2024) and ionizing photon production efficiency (H. Atek et al. 2024; A. Harshan et al. 2024; M. J. Hayes et al. 2025; M. Llerena et al. 2025; S. Mascia et al. 2024; C. Simmonds et al. 2024) of the galaxy population at redshift $z > 9$. These observations have found a larger abundance of UV-bright galaxies than previously thought, and an increased ionizing production efficiency for UV-faint, low-mass sources. Despite these advances, a full picture of reionization history requires constraints on the escape fraction. Yet, measurements of $f_{\text{esc}}^{\text{LyC}}$ during the EoR are impeded by the fraction of remaining neutral hydrogen in the IGM, a small amount of which is nevertheless sufficient to absorb all LyC photons that escape from the galaxies where they were produced (A. K. Inoue et al. 2014). As a result, the escape fraction cannot be constrained directly at this epoch, leaving uncertainties on the physical mechanisms enabling galaxies to reionize the Universe. Changes of the $f_{\text{esc}}^{\text{LyC}}$ dependence on observed galaxy properties can significantly alter the reionization history (e.g., S. L. Finkelstein et al. 2019; R. P. Naidu et al. 2020; Y.-H. Lin et al. 2024). Thus, determining the value of this parameter and how it varies across time and galaxy populations is vital to understanding the EoR.

To circumvent limitations posed by the IGM absorption in the early Universe, observations have turned to star-forming galaxies at lower redshift ($z < 4$) to search for LyC-emitting galaxies, and ultimately identify the properties that facilitate LyC escape (N. Bergvall et al. 2006; E. Leitet et al. 2013; S. Borthakur et al. 2014; R. E. Mostardi et al. 2015; S. de Barros et al. 2016; Y. I. Izotov et al. 2016a, 2016b, 2018a, 2018b, 2021, 2022; A. E. Shapley et al. 2016; E. Vanzella et al. 2016; M. J. Rutkowski et al. 2017; T. E. Rivera-Thorsen et al. 2019; B. Wang et al. 2019; Z. Ji et al. 2020; S. R. Flury et al. 2022a; A. Saxena et al. 2022; X. Xu et al. 2022; A. Citro et al. 2025; N. Roy et al. 2024). These surveys offer the potential to explore and evaluate the galaxy properties linked to ionizing radiation escape. Low-redshift surveys have shown that the detection of LyC emission is generally associated with galaxy properties characteristic of young, intensely star-forming, compact, and low-metallicity galaxies (e.g., Y. I. Izotov et al. 2016b; S. R. Flury et al. 2022b, 2025; C. A. Carr et al. 2025), although see N. Roy et al. (2024). While such galaxy properties increase the chance of observing LyC emission, they are not sufficient, and the line-of-sight distribution of neutral hydrogen (H I) gas strongly impacts LyC escape in a given direction. In particular, studies have shown that lines of sight with lower H I covering fraction are more favorable to LyC escape (S. Gazagnes et al. 2018, 2020;

C. C. Steidel et al. 2018; A. Saldana-Lopez et al. 2022), and that the global distribution of H I could also play a role in the escape of ionizing emission (A. Le Reste et al. 2024; F. Leclercq et al. 2024).

Several quantities have been investigated as proxies to the LyC emission from galaxies, such as the ratio between the [O III]λ5008 and [O II]λ3727 emission lines $O_{32}$, characterizing the ionization state of a galaxy (A. E. Jaskot & M. S. Oey 2013; J. Chisholm et al. 2018; Y. I. Izotov et al. 2018b; K. Nakajima et al. 2020; S. R. Flury et al. 2022b). The UV $\beta$ slope is also often used, as it simultaneously traces dust content and the age of stellar populations, both thought to be linked to the production and escape of ionizing photons (E. Zackrisson et al. 2013; J. Chisholm et al. 2022). The properties of the Ly$\alpha$ line have been found to be among one of the best predictors of the escape fraction of ionizing radiation (A. Verhamme et al. 2015; S. R. Flury et al. 2022b). In galaxies, Ly$\alpha$ is commonly emitted following ionization of hydrogen by LyC emission, through recombination. Similarly to LyC radiation, Ly$\alpha$ emission is strongly impacted by neutral hydrogen and dust. Furthermore, Ly$\alpha$ emission from galaxies has been observed during the EoR (e.g., J. Witstok et al. 2025; C. Witten et al. 2024; M. Tang et al. 2024), making it a particularly interesting tracer of LyC emission, as it could be used to identify the very galaxies that reionized the Universe. However, most stand-alone galaxy properties, including Ly$\alpha$ observables, are insufficient to accurately predict the escape fraction of LyC photons (S. R. Flury et al. 2022a). Recently, multivariate analysis has shown a tremendous improvement in the prediction of the escape fraction (N. Choustikov et al. 2024; A. E. Jaskot et al. 2024a). Such multivariate models are, for the first time, providing calibrations that can be used to indirectly estimate the LyC escape fraction of galaxies during the EoR, and predict the relative contributions of different galaxy populations to cosmic reionization (A. E. Jaskot et al. 2024b; S. Mascia et al. 2024).

Nevertheless, and despite recent improvements on $f_{\text{esc}}^{\text{LyC}}$ predictions using multivariate models, such predictions still present a large scatter: up to 2 dex for predictions based on single variables (S. R. Flury et al. 2022b), and down to 0.31 dex for the best available multivariate models (A. E. Jaskot et al. 2024a). Studies have remained limited by several factors, including the fact that galaxies are objects with complex, three-dimensional structures. Few galaxies currently have available sub-kiloparsec-scale LyC observations. Notable examples of galaxies with resolved LyC emission are the closest confirmed LyC emitter Haro 11 (N. Bergvall et al. 2006; L. Komarova et al. 2024), the lensed $z \sim 2.4$ Sunburst Arc (H. Dahle et al. 2016; T. E. Rivera-Thorsen et al. 2019; K. J. Kim et al. 2023; M. R. Owens et al. 2024), the galaxy with the highest $f_{\text{esc}}^{\text{LyC}}$ measured, J1316+2614 (R. Marques-Chaves et al. 2024), and one of the highest redshift ($z = 3.794$) LyC emitter, Ion1 (Z. Ji et al. 2025). In those objects, the LyC emission and escape fraction have been found to vary spatially. In the Sunburst Arc in particular, the majority of escaping LyC emission stems from a single, compact region ($\sim 8$ pc) within the galaxy (U. Meštrić et al. 2023).

High-resolution hydrodynamic simulations provide a complementary view, revealing that LyC and Ly$\alpha$ escape are both highly anisotropic and time-variable. Several studies have linked bursty feedback to intermittently higher escape fractions, albeit with a complex dependence on halo mass, star
formation history, and ISM structure (N. Y. Gnedin et al. 2008; J. H. Wise & R. Cen 2009; H. Yajima et al. 2011; T. Kimm & R. Cen 2014; J. H. Wise et al. 2014; R. Cen & T. Kimm 2015; M. Trebitsch et al. 2017; K. S. S. Barrow et al. 2020; X. Ma et al. 2020, 2021). Given the large inherent scatter and a desire to understand the global budget for reionization, large-volume simulations (10–100 cMpc in size) have also been employed to extend the statistical reach of LyC escape studies, e.g., FiBy (J.-P. Paardekooper et al. 2015), SPHINX (J. Rosdahl et al. 2018, 2022; H. Katz et al. 2023), THESAN (J. Y. C. Yeh et al. 2023), and IllustrisTNG (I. Kostyuk et al. 2023). Monte Carlo Lyα radiative transfer postprocessing predicts LyC connections between the properties of young (≤5 Myr) star clusters and galaxy-scale processes, including H I covering fractions, dust, and velocity effects, and spatial spectral features (A. Smith et al. 2019, 2022; J. Blaizot et al. 2023; A. Bhagwat et al. 2024). Beyond this, semi-analytical, numerical experiments, and cloud simulations reveal insights about the porosity in regulating LyC and Lyα escape (C. Carr et al. 2021; K. Kakiichi & M. Gronke 2021; S. H. Menon et al. 2025).

Despite this progress, simulations have still not reached consensus on what feedback prescriptions and numerical resolutions are required to properly predict LyC and Lyα escape, providing further motivation for obtaining constraints from high-resolution observations. Currently, only globally integrated galaxy properties of LyC emitters have been measured in a statistically robust manner, even though theoretical and observational frameworks indicate LyC escape to be highly anisotropic.

The Lyα and Continuum Origins Survey (LaCOS) has been proposed to address the current lack of information on how resolved galaxy properties influence LyC emission and escape. LaCOS consists of HST photometry of 42 galaxies part of the extended Low redshift Lyman Continuum Survey (LzLCS+; Y. I. Izotov et al. 2016a; S. R. Flury et al. 2022a), obtained in five filters covering the rest-frame UV and optical. The LaCOS images have an ~0.01 point-spread function (PSF), corresponding to a physical scale of ~400 pc, allowing one to relate sub-kiloparsec-scale properties to the LyC output of galaxies. In particular, LaCOS allows for imaging of the Lyα emission in these galaxies, thought to trace LyC escape. Here, we present the survey, data, and results from photometry. In Section 2, we detail the sample selection and observations, and present the data products that are being released to the community. In 3, we describe the HST data reduction procedure and present the optical maps of the galaxies in the sample. Section 4 presents the technique employed to obtain Lyα maps of the galaxies in the sample. In Section 5, we compare global photometry values to those obtained with the Cosmic Origins Spectrograph (COS), and examine whether any trend can be found between sub-kiloparsec-scale galaxy properties and the line of sight $f_{\text{esc}}^{\text{LyC}}$ in LaCOS galaxies. We summarize results in Section 6. Additional investigations using LaCOS data include studies of the link between Lyα halo extent and $f_{\text{esc}}^{\text{LyC}}$ (A. Saldana-Lopez et al. 2025), between quantified morphological properties, galaxy merger properties, and LyC production and escape (A. Le Reste et al. 2025, in preparation), and will use UV $\beta$ spectral slope maps to investigate the stellar population and dust properties of LaCOS galaxies (I. Jung et al. 2025, in preparation).

This work assumes a standard flat $\Lambda$CDM cosmology with $H_0 = 70 \text{ km s}^{-1} \text{ Mpc}^{-1}$, and $\Omega_m = 0.3$.

## 2. LaCOS

### 2.1. Sample Selection

The LaCOS sample was primarily built from the LzLCS sample, the largest sample of galaxies observed uniformly in LyC at low-redshift (66 galaxies, S. R. Flury et al. 2022a), in an effort to understand how the resolved properties of galaxies connect with LyC emission and escape. The LzLCS targets were selected from an initial sample of galaxies with available Sloan Digital Sky Survey (SDSS) spectroscopy and Galaxy Evolution Explorer (GALEX) coverage. Specifically, the LzLCS sample selection targeted nearby galaxies ($z \sim 0.3$) with no sign of active galactic nuclei activity, relatively high ionization states traced via $O_{32}$, high star formation rate surface densities $\Sigma_{SFR}$, or blue UV continua (via their spectral slope index $\beta$). Nevertheless, galaxies were required to fulfill only one of the criteria above to be included in the sample, resulting in a relatively wide range of properties being covered. A major goal of the LaCOS survey is to obtain resolved Lyα maps of objects with LyC observations, to identify whether Lyα morphology correlates in any way with LyC emission properties. To that effect, the LaCOS sample was constructed from the LzLCS sample, selecting galaxies for which Lyα imaging could be obtained via the effective narrowband technique. This imaging method, presented in M. Hayes et al. (2005), allows the construction of emission line maps from observations with two Solar Blind Channel (SBC) long-pass filters. One filter samples the emission line and the surrounding UV continuum, and the other, only the UV continuum. The emission line map, in this case Lyα, can be retrieved by scaling and subtracting one image from the other. This technique makes imaging of Lyα possible only within a narrow redshift range; for that reason, only LzLCS galaxies with redshift $z < 0.32$ are selected. This selection reduced the size of the sample from 66 to 41 galaxies. One additional galaxy from the literature with available archival imaging was added (Y. I. Izotov et al. 2016a), resulting in a sample of 42 galaxies.

The final LaCOS sample includes 22 galaxies detected in LyC with a $>2\sigma$ significance (as compared to 35 galaxies in the LzLCS sample, see S. R. Flury et al. 2022a), and 20 galaxies with upper limits. The redshift selection thus resulted in a near-even split of LyC emitters and nonemitters, required for statistically robust estimations of the impact of galaxy properties on LyC emission and escape.

In Figure 1, we present histograms for the properties of LzLCS and LaCOS galaxies, as derived in S. R. Flury et al. (2022a). The distribution of properties and their means are extremely similar for the LzLCS and LaCOS samples. Additionally, we run Kolmogorov-Smirnov tests (using the `kstest` implementation in the Python package `SciPy`) to verify whether the LaCOS sample is indeed consistent with being drawn from the same distribution as the LzLCS sample. We test each property presented in Figure 1, and require a standard $p$-value threshold $p < 0.05$ to reject the null hypothesis, which states that the two samples are drawn from the same distribution. Given that $p > 0.84$ for all properties considered in Figure 1, we cannot reject the null hypothesis. The simple redshift cut operated for the LaCOS sample
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Histogram of properties for the LaCOS (blue) and LzLCS (gray) samples. Vertical bars show the average properties for the two samples (calculated in real space for base 10 log values). The distribution of properties for the LaCOS sample is similar to that of the full LzLCS sample.</figcaption></figure>

selection therefore selects galaxies that are representative of
the full LzLCS sample.

## 2.2. Data

The data for the LaCOS program consist of observations in three optical bands with HST WFC3/UVIS filters F850LP, F547M, and F438W (with a 162" × 162" field of view and 0.04 pixel scale), and two UV bands with HST Advanced Camera for Surveys (ACS)/SBC filters F165LP and F150LP (with 34.6 × 30.5 field of view and 0.032 pixel scale). With the redshift cut operated to select the sample, the Lyα line falls within the HST ACS/SBC F150LP filter, while the F165LP filter samples the UV stellar continuum. The three optical filters were chosen to sample the Balmer break, optical stellar continuum, and the broadband continuum including the Hα line for some galaxies. The bulk of data was obtained as part of HST GO program 17069 (PIs: Hayes, Scarlata).

Three of the selected galaxies (J092532, J113304, and J124835) had high-quality archival data available, which were used to complete the sample. For J124835 and J092532, programs 14131 and 14466 (PI: Orlitova) provided observations in all aforementioned instruments and filter combinations, with the exception of observations with filter F850LP, which were executed with the instrument ACS/Wide Field Camera (WFC; with a 202" × 202" field of view and 0.05 pixel scale), instead of WFC3/UVIS. Additionally, the ACS/SBC F150LP filter observations for galaxy J113304 were obtained by HST program 11107 (PI: Heckman).

Exposure times for the different targets and filters, as well as
detector temperature for SBC observations, are compiled in
Table 3 in Appendix C. We present an illustration of the HST
photometric coverage relative to the COS UV and optical
SDSS spectra for galaxy J172010 in Figure 2.

We adopt the values described in S. R. Flury et al. (2022a)
and A. Saldana-Lopez et al. (2022) for the various galaxy
properties presented in this manuscript. These include the
properties shown in Figure 1, and the LyC escape fractions,
which are derived from fits to the UV continuum spectral
energy distribution. Additionally, following the convention

adopted in LzLCS studies, galaxies are considered to be
detected in LyC only if they reach a sufficiently low
probability of the measured counts originating from the
background, *P* < 0.02275 (2*σ*).

## 2.3. Data Availability

The data products for the HST observation of LaCOS galaxies (including archival data) are being released at the Barbara A. Mikulski Archive for Space Telescopes (MAST) as a High Level Science Product, with the following DOI:10.17909/j4qd-ev76.<fnref n="27" /> The released data products include HST images in all individual instrument and filters combination (ACS/SBC F150LP, F165LP, and WFC3/UVIS F438W, F547M, and F850LP, or ACS/WFC F850LP). Additionally, it contains Lyα maps and UV continuum maps for all LaCOS galaxies. Finally, the data release contains a table presenting galaxy properties and photometric measurements, and in particular, photometric Lyα measurements, r₅₀ measured in the F165LP filter, star formation rate surface density, and aperture photometry in all filters, extracted in a circular aperture centered on the galaxy coordinates, with radius 3 × r₅₀. In the following, we present the data reduction methods employed to create the imaging data products.

## 3. HST Data Reduction

Our data reduction approach is inspired by those described
in A. Runnholm et al. (2023) and J. Melinder et al. (2023), but
has been modified to suit the data available for the LaCOS
program. The steps to this approach are described below.

We start the reduction with *flt* and *flc* frames
downloaded from the MAST archive. These frames have
undergone preliminary data reduction with the standard HST
reduction pipelines (including bias, initial dark current
subtraction, flat-fielding, and CTE correction) using the latest
calibration files.

<footnote n="27">²⁷ Also accessible via https://archive.stsci.edu/hlsp/lacos/.</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Illustration of LaCOS HST photometry coverage for galaxy J172010 ($z = 0.29$). The leftmost panel shows a Ly$\alpha$ map of the galaxy, with COS (white) and SDSS (gray) extraction apertures overlaid. The panel shows Ly$\alpha$ flux values between $5 \times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ (in black) and $1 \times 10^{-16}$ erg s$^{-1}$ cm$^{-2}$ (in white), displayed on a logarithmic scale. The middle panel shows the COS spectrum for wavelengths overlapping with ACS/SBC filter coverage, with filter throughput overlaid. The rightmost panel shows the SDSS spectrum with WFC3/UVIS filter responses overlaid.</figcaption></figure>

### 3.1. Dark Current and Background Subtraction from SBC Frames

SBC data can be impacted by dark current, which manifests as an elevated, spatially varying background in frames with high detector temperature. While general SBC guidelines have recommended dark current correction to be performed on frames when the detector temperature surpasses 25°C, SBC dark rates have recently been shown to increase before this threshold is reached (A. M. Guzman & R. J. Avila 2024). Since dark current could mimic the extended Ly$\alpha$ haloes we are trying to image, removing any dark current contribution from the science frame is imperative. After inspection of the science frames, we set a threshold temperature of $T > 22^\circ C$ for implementation of the residual dark current subtraction. The method is described in A. Runnholm et al. (2023), but in short, we use a collection of dark current files from the same cycles as the observations to simultaneously implement a fit to the sky background, and if necessary, dark current pattern in SBC frames using $\chi^2$ minimization. Prior to fitting, the galaxy is masked from the frames using a circular 200 pixel radius mask. From the collection of dark current files, the best-fitting dark frame is identified and scaled to produce a dark current image. We combine this fitted dark current frame with the sky background and subtract those from the data. If the detector temperature is below the threshold temperature ($T < 22^\circ C$), only the sky background is subtracted.

In some instances, the SBC frames still show a gradient background after sky background and/or dark current subtraction. This is a common issue in ACS exposures, linked to the reflection of solar light on Earth at low limb angles (J. Biretta et al. 2003; L. J. Prichard et al. 2022). To correct for this, we model the background as a 2D order 2 polynomial and subtract it from the exposure. The modeling is performed by masking the galaxy using a circular 200 pixel radius mask, and using `astropyPolynomial2D` model and the Levenberg-Marquardt fitting algorithm. We inspect each background-subtracted frame visually to ensure the background is fully removed before proceeding with further data reduction.

### 3.2. Cosmic-Ray Rejection from UVIS and WFC Files

Cosmic rays strongly impact WFC3/UVIS frames. Since we obtained only two UVIS frames per filter, drizzling does

not remove all cosmics from the data. To remedy this issue, we run a Python implementation of L.A.Cosmic (P. G. van Dokkum 2001) on the `flc` frames prior to drizzling to reject cosmic rays. All frames are inspected after cosmic-ray removal. Cosmetics are successfully removed in most cases, but in a few instances, noncontaminated parts of the galaxy are flagged as cosmic rays. We manually unflag parts of the frame that were wrongly flagged when necessary.

### 3.3. Additional Background Correction for UVIS Frames

Upon further inspection of the frames, we identify offsets between the left and right halves of Chip 2 for the WFC3/UVIS `flc` frames. This discontinuity is caused by differences in bias for the amplifiers, and impacts 18% of the `flc` frames. Since galaxies are at the center of Chip 2, this discontinuity could affect the fluxes and morphological parameters in the filters impacted. To correct for this, we model and subtract the background separately for the two halves of the chip in the frames impacted. We perform this correction on frames after cosmic-ray rejection. Specifically, we use `photutils` to detect sources and mask them from frames prior to calculating the median background in each half of the chip and subtracting a constant from each frame. A handful of frames present additional structure in their background. In those, we use the `photutils` function `Background2D` to model a spatially varying background. All UVIS frames are visually inspected to ensure the background is subtracted correctly.

### 3.4. World Coordinate System Registration and Frame Alignment

We use the custom image alignment tool `CROCOA`$^{28}$ developed for the data reduction in A. Runnholm et al. (2023). This tool enables fine alignment of frames even in the absence of stars, which is typically the case for the SBC UV frames. Shortly, the code searches for the shifts and rotation in the coordinates that maximize the correlation of pixel intensity across different frames. The frames are first aligned in individual filters, and a second run of `CROCOA` is performed on the pre-aligned frames across the different filters. We visually assess the alignment, adapt the parameters, and repeat

<footnote>$^{28}$ Publicly available on https://github.com/runnholm/CROCOA.</footnote>
the process when necessary to ensure all frames are aligned prior to drizzling.

### 3.5. Drizzling and Additional Background Subtraction

Once the UVIS and ACS frames are pre-aligned, the frames for individual filters are registered and stacked together to a final pixel scale of 0.04 (the native WFC3 pixel scale) using the HST drizzling software (drizzlepac). For all filters, we produce maps 600 × 600 pix² (24" × 24") in size. Since LaCOS galaxies are compact ($r_{50} \sim 0.2$), these maps fully enclose the emission in all filters for all galaxies. Drizzling also produces weight maps (or inverse variance), which we use to estimate pixel noise.

After drizzling the frames, we subtract any additional background that may be present from the images. We visually inspect the background for all galaxies in each filter individually, and determine the best subtraction scheme, considering several options, including no background, a constant background, and 2D polynomials with orders from 1 up to 3. Each galaxy is first masked from the frame using a circular mask with a 200 pixel radius (8"). This mask is sufficiently large so that it excludes extended emission from the galaxies, including the Lyα emission in F150LP images. The masked image, containing only the background, is fit with the function thought to best represent the background. The best-fit to the background is then subtracted from the science frame, and the background-subtracted image is visually inspected to ensure the subtraction was successful. For UVIS filters, the background is, in most cases, well described by an order 1 2D polynomial. For SBC frames, most of the galaxies have a background well approximated by a constant, due to prior background subtraction operated on individual frames (see Section 3.1).

### 3.6. SBC Map Reweighting

Drizzling SBC frames to a larger pixel scale than native leads to noise in drizzled SBC frames being overestimated (see A. Runnholm et al. 2023). Thus, the SBC frame weights are recalculated to take into account the correlation of noise across several pixels. To better estimate the noise, we estimated the sky background from the drizzled frames, and then multiplied sky background frames by the factor R (A. S. Fruchter & R. N. Hook 2002), where, given our choice of 0.04 for the final pixel scale pixscale:

$$R = \frac{1}{1 - \frac{1}{3r}}, \qquad (1)$$

with $r = 1/pixscale = 1.25$.

### 3.7. PSF Matching

The drizzled frames are matched to a common PSF to enable comparison of the final images in different filters. Standard PSF matching techniques have problems with matching data sets containing SBC far-ultraviolet (FUV) images, due to the extended wings in the SBC PSF. We use the matching method and PSF models from J. Melinder et al. (2023), developed for accurate matching of SBC imaging to optical HST imaging. The PSF models for UV filters are derived for a general case from archival observations of stellar cluster NGC 6681 (J. Melinder et al. 2023). The images are

matched to a common PSF, which is constructed from all of the filters to be the broadest PSF at any given radius. In the central pixels, the PSF is dominated by the contribution from the F850LP filter, while in the outer pixels, the PSF contribution is dominated by the UV filters. After PSF matching, the frames are corrected for the Milky Way extinction using values reported in S. R. Flury et al. (2022a), and the E. L. Fitzpatrick (1999) extinction law.

We produce red, green, and blue (RGB) optical color-composite images for each galaxy, presented in Figure 3, with the F850LP filter in red, F547M in green, and F438W in blue. Each frame is scaled to its own maximum prior to producing the composite image, to highlight structural differences across different bands. The panels display the central 20 × 20 kpc area around the target galaxy and are sorted by increasing $f_{esc}^{LyC}$. While most galaxies in the LaCOS sample are physically compact across all filters, a few of them exhibit large-scale features extending beyond the 20 kpc × 20 kpc frames presented in Figures 3 and 4, which tend to be more visible in redder filters. In Figure 13 in Appendix C, we present additional larger-scale optical RGB color-composite images for three galaxies with emission extending beyond the 20 kpc cutouts in Figure 3. The resolution reached by HST observations makes it immediately noticeable that LaCOS galaxies have a variety of morphologies, most being irregular, and with several objects having apparent tidal features. We will explore the link between galaxy morphology, merger interaction, and LyC emission in a forthcoming manuscript (A. Le Reste et al. 2025, in preparation).

## 4. Lyα Maps

### 4.1. Methods

With five photometric bands available, and in the absence of narrowband imaging covering the Hα emission, we cannot employ commonly used photometric spectral energy distribution (SED) fitting software (such as the Lyman Alpha eXtraction Software, or LaXs, M. Hayes et al. 2009; J. Melinder et al. 2023) to produce Lyα emission maps. Nevertheless, we have access to low-resolution spectra of Lyα and the UV continuum, obtained with HST/COS for all galaxies in the sample, and to imaging in UV filters covering the Lyα line and the off-line UV continuum. Thus, we can use available imaging and spectra to derive maps reproducing the global Lyα spectroscopic properties. Note that one galaxy (J124835) has additional photometry. We present a comparison of Lyα maps obtained with photometric SED fitting using LaXs and the method outlined below in Appendix C. Generally, the morphology of the Lyα and UV continuum maps, as well as the UV continuum flux densities derived through the method presented here and via photometric SED fitting with LaXs agree; however, the Lyα flux (and thus EW) differs significantly, with LaXs values being about half those of the LaCOS and COS measurements. This underestimation of Lyα flux has been observed previously with LaXs; thus, the discrepancy with our measurement is not a source of concern for the quality of data reduction, and we proceed with the method described below.

The LaCOS observations have been designed for the F150LP filter to cover both the Lyα emission and UV continuum, while the F165LP covers solely the UV continuum. The flux density measured in the F150LP frames can
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Optical RGB color-composite images of LaCOS galaxies. The F850LP data is shown in red, F547M in green, and F438W in blue. The intensity is normalized to the maximum of the emission in each filter in order to highlight structural differences across different bands, but the bulk of the optical flux is in the F438W filter. The panels are ordered from left to right and top to bottom by increasing $f_{esc}^{LYC}$ (with value shown on the bottom right corner), and all display the galaxies on a scale of 20 kpc × 20 kpc, which encompasses the emission in all filters in most cases. Galaxies with emission extending beyond the scale presented in the panels are shown in full in Figure 13. LaCOS galaxies show very diverse morphologies and many exhibit signs of merger interaction.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Lyα maps of the LaCOS galaxies. The panels display a physical scale of 20 kpc × 20 kpc, and are ordered by increasing LyC escape fraction, displayed in the lower right corner of each panel. For all panels, the Lyα flux values are shown between 5 × 10⁻¹⁸ erg s⁻¹ cm⁻² (in black) and 2.5 × 10⁻¹⁶ erg s⁻¹ cm⁻² (in white), displayed on a logarithmic scale.</figcaption></figure>
thus be written as a function of the $Ly\alpha$ line flux $F_{Ly\alpha}$, the UV continuum flux density probed in the F150LP filter $f_{c,F150LP}$, the F165LP filter $f_{F165LP}$, and the F150LP bandwidth $BW_{F150LP} = 109.2$ Å:

$$f_{F150LP} = \frac{F_{Ly\alpha}}{BW_{F150LP}} + f_c, \quad \text{F150LP.} \quad (2)$$

Here, we introduce the bandpass normalization factor $\alpha$ that links the continuum flux density measured in both filters such that

$$f_c, \text{F150LP} \approx \alpha \cdot f_{F165LP}. \quad (3)$$

The $Ly\alpha$ images can therefore be obtained once the value of the bandpass normalization factor is known through

$$F_{Ly\alpha} = (f_{F150LP} - \alpha \cdot f_{F165LP}) \times BW_{F150LP}. \quad (4)$$

A simple way of solving for the value of $\alpha$ would be to match the $Ly\alpha$ photometric flux to the value estimated from the COS spectra. However, to limit possible issues posed by absolute flux calibration errors (theoretically on the order of 2.3%, W. J. Fischer et al. 2019), we instead match the $Ly\alpha$ image to have the same $Ly\alpha$ equivalent width (EW) within the COS aperture, as that measured in the COS spectrum. The EW depends on the relative flux between the $Ly\alpha$ line and UV continuum, and as such, is not impacted by flux calibration uncertainties on COS. However, we note that regardless of performing the match using the spectroscopic EW or $F_{Ly\alpha}$, the resulting $Ly\alpha$ images could still be impacted by zero-point uncertainties on ACS/SBC images (1.7% for F150LP, and 4.5% for F165LP, R. J. Avila et al. 2019), we thus take these uncertainties into account when deriving the error maps.

The $Ly\alpha$ EW can be expressed as a function of the $Ly\alpha$ flux and the continuum flux density value around the $Ly\alpha$ line $f_c$, $Ly\alpha$ as

$$\text{EW} = \frac{\int f_{Ly\alpha} d\lambda}{f_c, \text{Ly}\alpha} = \frac{F_{Ly\alpha}}{f_{c,Ly\alpha}}. \quad (5)$$

In the photometric case $f_{c,Ly\alpha} = f_{c,F150LP}$, so we combine Equations (3) and (4) to write

$$\text{EW}_{\text{phot}} = \left( \frac{1}{\alpha} \cdot \frac{f_{\text{F150LP}}}{f_{\text{F165LP}}} - 1 \right) \cdot \text{BW}_{\text{F150LP}}. \quad (6)$$

Since we have access to the value of the rest-frame $Ly\alpha$ equivalent width $EW_{\text{COS}}$ in the COS aperture (S. R. Flury et al. 2022a), we can solve for $\alpha$ by extracting photometry in the same aperture as that used to extract the COS spectra. To do so, we use a vignetted circular aperture with 2.5" diameter (see the example COS extraction aperture shown on the left panel of Figure 2, and the discussion in Appendix B). We center the aperture upon the brightest pixel in the F165LP image. We sum the photometric $Ly\alpha$ flux and the continuum flux density within the aperture to obtain a photometric equivalent width matched to that observed with COS,$^{29}$ $EW_{\text{COS,obs}} = (1+z) \cdot EW_{\text{COS}}$,

and derive the bandpass normalization factor $\alpha$ as

$$\alpha = \frac{f_{F150LP,ap}}{f_{F165LP,ap}} \cdot \frac{BW_{F150LP}}{EW_{\text{COS,obs}} + BW_{F150LP}}. \quad (7)$$

We use the $\alpha$ values (0.63–1.1) derived by matching the photometric and spectroscopic EW to derive $Ly\alpha$ maps using Equation (4), UV continuum maps using Equation (3), and EW maps using Equation (5). Error maps are derived via error propagation, taking into account the errors on $\alpha$, uncertainty on pixel flux, and zero-point uncertainties on photometry.

## 4.2. $Ly\alpha$ Maps

We present the $Ly\alpha$ maps obtained following the approach described in 4.1 in Figure 4. Additionally, we show EW maps in Figure 5, produced by smoothing the $Ly\alpha$ and UV continuum maps with a Gaussian kernel with 2.5 pixel full width at half-maximum (FWHM), and masking regions with $f_{\lambda,UV} < 1.5 \times 10^{-19} \text{ erg s}^{-1} \text{ cm}^{-2} \text{ Å}^{-1}$, prior to dividing the $Ly\alpha$ by the continuum maps. Similarly to Figure 3, all panels have a physical size of 20 kpc × 20 kpc, and are sorted from top to bottom and left to right by LyC escape fraction. Additionally, all maps are shown on the same logarithmic scales. The galaxies with the highest $f_{esc}^{LyC}$ tend to have brighter and more compact $Ly\alpha$ emission. Furthermore, galaxies with high $f_{esc}^{LyC}$ tend to have larger EW (≥50 Å), while non-LyC-emitting galaxies often show low and negative EW values, indicative of $Ly\alpha$ absorption, linked to the presence of neutral gas or dust on the line of sight. Nevertheless, the $Ly\alpha$ emission intensity, morphology, and extent vary strongly across the sample. Detailed analysis of the $Ly\alpha$ halo morphology and the relation with LyC output is presented in A. Saldana-Lopez et al. (2025). In the following, we explore the link between global $Ly\alpha$ photometry measured in the SBC maps and the LyC escape fraction.

In Table 1, we present galaxy properties, including measurements from $Ly\alpha$ photometry for the LaCOS sample. The redshift and $f_{esc}^{LyC}$ are obtained from S. R. Flury et al. (2022a). The $Ly\alpha$ properties, UV continuum flux density $f_{\lambda,UV}$ and $r_{50}$ values are derived through SBC photometry.

## 4.3. Comparison to Spectroscopic $Ly\alpha$ Measurements

In Figure 6, we compare the $Ly\alpha$ measurements obtained from the COS spectra to the global $Ly\alpha$ measurements from SBC photometry, specifically the $Ly\alpha$ luminosity, EW, and escape fraction $f_{esc}^{Ly\alpha}$. The $Ly\alpha$ flux is measured through curve-of-growth integration within circular apertures, each spaced by 5 pixels (0.2"). Using this approach, we measure the flux in increasingly larger apertures and set the aperture for integration when the flux reaches a plateau (with less than 0.1% fractional variation in flux), or to the radius where the $Ly\alpha$ flux reaches a maximum. This circular aperture is used to measure the $Ly\alpha$ flux, EW, and $f_{esc}^{Ly\alpha}$, and has an average radius of 173 pixels (6.9"). We calculate the rest-frame EW from the observed values, such that $EW = EW_{\text{obs}}/(1+z)$. The $Ly\alpha$ escape fraction $f_{esc}^{Ly\alpha}$ is obtained by dividing the observed photometric $Ly\alpha$ flux by the intrinsic $Ly\alpha$ flux. The latter value is estimated using dust-corrected $H\beta$ flux derived from

<footnote>$^{29}$ The spectroscopic $Ly\alpha$ flux and EW we use here for galaxy J092532 have been updated from the values presented in S. R. Flury et al. (2022a) to $F_{Ly\alpha} = 2.20 \pm 0.09 \times 10^{-14} \text{ erg s}^{-1} \text{ cm}^{-2}$ and EW = 74.6 ± 5.8 Å as an internal dust correction had been applied in Y. I. Izotov et al. (2016a) where the values were reported, which had not been taken into account previously.</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Rest-frame Lyα EW maps of the LaCOS galaxies. The panels display a physical scale of 20 kpc × 20 kpc, and are ordered by increasing LyC escape fraction, displayed in the lower right corner of each panel.</figcaption></figure>
<table><caption>**Table 1**
LaCOS Galaxy Properties and Photometric Measurements</caption><thead><tr><th>ID</th><th>z<sup>a</sup></th><th>f<sub>esc</sub><sup>Lyα</sup></th><th>L<sub>Lyα</sub> (10<sup>42</sup> erg s<sup>-1</sup>)</th><th>EW<sub>Lyα</sub> (Å)</th><th>f<sub>esc</sub><sup>Lyα</sup></th><th>f<sub>λUV</sub> (10<sup>-16</sup> erg s<sup>-1</sup> cm<sup>-2</sup> Å<sup>-1</sup>)</th><th>r<sub>50</sub> (kpc)</th><th>Σ<sub>SFR</sub> (M<sub>☉</sub> yr<sup>-1</sup> kpc<sup>-2</sup>)</th></tr></thead><tbody><tr><td>J011309</td><td>0.31</td><td>0.02<sup>+0.02</sup><sub>-0.01</sub></td><td>4.84 ± 0.33</td><td>47.8 ± 3.6</td><td>0.65 ± 0.12</td><td>2.56 ± 0.09</td><td>0.8 ± 0.09</td><td>1.06 ± 0.37</td></tr><tr><td>J012910</td><td>0.28</td><td>&lt;0.007</td><td>5.4 ± 0.44</td><td>49.7 ± 4.6</td><td>0.23 ± 0.02</td><td>3.45 ± 0.15</td><td>0.9 ± 0.08</td><td>2.4 ± 0.48</td></tr><tr><td>J072326</td><td>0.3</td><td>&lt;0.004</td><td>3.16 ± 0.18</td><td>55.2 ± 3.7</td><td>0.23 ± 0.02</td><td>1.57 ± 0.05</td><td>0.7 ± 0.09</td><td>2.69 ± 0.75</td></tr><tr><td>J081409</td><td>0.23</td><td>&lt;0.008</td><td>0.26 ± 0.22</td><td>1.3 ± 1.1</td><td>0.01 ± 0.01</td><td>10.5 ± 0.12</td><td>1.8 ± 0.07</td><td>0.76 ± 0.08</td></tr><tr><td>J082652</td><td>0.3</td><td>&lt;0.009</td><td>1.45 ± 0.44</td><td>67.1 ± 24.8</td><td>0.14 ± 0.04</td><td>0.59 ± 0.13</td><td>0.8 ± 0.09</td><td>1.64 ± 0.45</td></tr><tr><td>J090918</td><td>0.28</td><td>0.49<sup>+0.42</sup><sub>-0.23</sub></td><td>3.47 ± 0.34</td><td>202.1 ± 47.8</td><td>0.22 ± 0.03</td><td>0.54 ± 0.12</td><td>0.5 ± 0.09</td><td>6.15 ± 2.15</td></tr><tr><td>J091113</td><td>0.26</td><td>0.02<sup>+0.02</sup><sub>-0.01</sub></td><td>6.18 ± 0.25</td><td>61.8 ± 2.9</td><td>0.14 ± 0.01</td><td>3.74 ± 0.09</td><td>0.6 ± 0.08</td><td>9.58 ± 2.43</td></tr><tr><td>J091207</td><td>0.25</td><td>&lt;0.008</td><td>1.71 ± 0.2</td><td>55.2 ± 7.4</td><td>0.08 ± 0.01</td><td>1.34 ± 0.09</td><td>1.4 ± 0.08</td><td>1.05 ± 0.14</td></tr><tr><td>J091703</td><td>0.3</td><td>0.16<sup>+0.07</sup><sub>-0.06</sub></td><td>5.91 ± 0.42</td><td>44.9 ± 3.5</td><td>0.18 ± 0.02</td><td>3.49 ± 0.12</td><td>0.5 ± 0.09</td><td>10.92 ± 3.69</td></tr><tr><td>J092532</td><td>0.3</td><td>0.09<sup>+0.02</sup><sub>-0.01</sub></td><td>5.08 ± 0.16</td><td>80.7 ± 3.0</td><td>0.14 ± 0.01</td><td>1.66 ± 0.04</td><td>0.6 ± 0.09</td><td>9.45 ± 2.73</td></tr><tr><td>J092552</td><td>0.31</td><td>&lt;0.004</td><td>1.29 ± 0.19</td><td>30.1 ± 4.6</td><td>0.15 ± 0.02</td><td>1.01 ± 0.05</td><td>1.2 ± 0.09</td><td>0.65 ± 0.16</td></tr><tr><td>J093355</td><td>0.29</td><td>0.27<sup>+0.11</sup><sub>-0.11</sub></td><td>5.26 ± 0.14</td><td>286.2 ± 19.1</td><td>0.4 ± 0.04</td><td>0.53 ± 0.03</td><td>0.7 ± 0.09</td><td>2.77 ± 0.78</td></tr><tr><td>J095236</td><td>0.32</td><td>0.04<sup>+0.02</sup><sub>-0.01</sub></td><td>3.05 ± 0.44</td><td>35.6 ± 5.5</td><td>0.28 ± 0.05</td><td>1.96 ± 0.11</td><td>1.1 ± 0.09</td><td>0.89 ± 0.2</td></tr><tr><td>J095700</td><td>0.24</td><td>&lt;0.001</td><td>-0.99 ± 0.13</td><td>-10.9 ± 1.4</td><td>-0.02 ± 0.0</td><td>4.02 ± 0.06</td><td>8.0 ± 0.23</td><td>0.07 ± 0.0</td></tr><tr><td>J095838</td><td>0.3</td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>2.21 ± 0.25</td><td>89.3 ± 13.7</td><td>0.09 ± 0.01</td><td>0.65 ± 0.07</td><td>1.4 ± 0.09</td><td>1.2 ± 0.17</td></tr><tr><td>J105331</td><td>0.25</td><td>0.01<sup>+0.01</sup><sub>-0.0</sub></td><td>4.77 ± 0.48</td><td>17.5 ± 1.8</td><td>0.1 ± 0.01</td><td>11.2 ± 0.21</td><td>0.8 ± 0.08</td><td>7.03 ± 1.43</td></tr><tr><td>J110452</td><td>0.28</td><td>&lt;0.011</td><td>3.44 ± 0.16</td><td>63.9 ± 3.6</td><td>0.49 ± 0.05</td><td>1.71 ± 0.05</td><td>0.9 ± 0.08</td><td>0.86 ± 0.25</td></tr><tr><td>J112224</td><td>0.3</td><td>0.03<sup>+0.06</sup><sub>-0.02</sub></td><td>3.33 ± 0.4</td><td>232.8 ± 69.6</td><td>0.28 ± 0.04</td><td>0.37 ± 0.1</td><td>0.6 ± 0.09</td><td>3.23 ± 1.01</td></tr><tr><td>J113304</td><td>0.24</td><td>0.02<sup>+0.02</sup><sub>-0.01</sub></td><td>4.95 ± 0.32</td><td>53.3 ± 3.9</td><td>0.43 ± 0.04</td><td>4.27 ± 0.15</td><td>1.0 ± 0.08</td><td>1.21 ± 0.25</td></tr><tr><td>J115855</td><td>0.24</td><td>0.07<sup>+0.03</sup><sub>-0.02</sub></td><td>7.91 ± 0.3</td><td>50.7 ± 2.1</td><td>0.22 ± 0.01</td><td>7.03 ± 0.14</td><td>0.7 ± 0.08</td><td>7.12 ± 1.62</td></tr><tr><td>J115959</td><td>0.27</td><td>0.04<sup>+0.07</sup><sub>-0.02</sub></td><td>3.81 ± 0.2</td><td>168.9 ± 17.9</td><td>0.35 ± 0.03</td><td>0.8 ± 0.07</td><td>0.8 ± 0.08</td><td>1.66 ± 0.41</td></tr><tr><td>J120934</td><td>0.22</td><td>&lt;0.013</td><td>5.84 ± 0.27</td><td>54.8 ± 2.9</td><td>0.19 ± 0.01</td><td>6.19 ± 0.16</td><td>0.6 ± 0.07</td><td>9.41 ± 2.41</td></tr><tr><td>J121915</td><td>0.3</td><td>0.01<sup>+0.02</sup><sub>-0.0</sub></td><td>1.99 ± 0.4</td><td>44.5 ± 10.0</td><td>0.21 ± 0.04</td><td>1.16 ± 0.11</td><td>1.0 ± 0.09</td><td>0.99 ± 0.25</td></tr><tr><td>J124033</td><td>0.28</td><td>&lt;0.011</td><td>1.87 ± 0.2</td><td>37.8 ± 4.4</td><td>0.11 ± 0.01</td><td>1.53 ± 0.07</td><td>0.7 ± 0.09</td><td>4.02 ± 1.07</td></tr><tr><td>J124423</td><td>0.24</td><td>&lt;0.015</td><td>6.15 ± 0.35</td><td>71.1 ± 5.0</td><td>0.11 ± 0.01</td><td>4.06 ± 0.17</td><td>1.4 ± 0.08</td><td>3.03 ± 0.35</td></tr><tr><td>J124835</td><td>0.26</td><td>0.05<sup>+0.04</sup><sub>-0.03</sub></td><td>10.96 ± 0.14</td><td>113.9 ± 2.0</td><td>0.4 ± 0.02</td><td>3.56 ± 0.04</td><td>0.7 ± 0.08</td><td>5.18 ± 1.19</td></tr><tr><td>J125503</td><td>0.31</td><td>&lt;0.009</td><td>5.18 ± 0.32</td><td>63.9 ± 4.8</td><td>0.31 ± 0.03</td><td>1.96 ± 0.08</td><td>1.6 ± 0.09</td><td>0.64 ± 0.09</td></tr><tr><td>J125718</td><td>0.31</td><td>&lt;0.014</td><td>2.7 ± 0.18</td><td>65.2 ± 5.2</td><td>0.12 ± 0.01</td><td>0.99 ± 0.04</td><td>0.7 ± 0.09</td><td>4.11 ± 1.08</td></tr><tr><td>J130559</td><td>0.32</td><td>0.18<sup>+0.08</sup><sub>-0.06</sub></td><td>1.64 ± 0.3</td><td>182.9 ± 69.0</td><td>0.14 ± 0.03</td><td>0.21 ± 0.07</td><td>0.6 ± 0.09</td><td>3.8 ± 1.38</td></tr><tr><td>J131037</td><td>0.28</td><td>0.02<sup>+0.02</sup><sub>-0.01</sub></td><td>3.55 ± 0.24</td><td>50.4 ± 3.9</td><td>0.18 ± 0.02</td><td>2.18 ± 0.08</td><td>0.6 ± 0.09</td><td>5.49 ± 1.64</td></tr><tr><td>J131419</td><td>0.3</td><td>&lt;0.001</td><td>2.31 ± 0.49</td><td>13.0 ± 2.8</td><td>0.06 ± 0.01</td><td>4.9 ± 0.14</td><td>2.0 ± 0.09</td><td>0.87 ± 0.09</td></tr><tr><td>J131904</td><td>0.32</td><td>&lt;0.002</td><td>0.74 ± 0.16</td><td>30.3 ± 6.8</td><td>0.03 ± 0.01</td><td>0.57 ± 0.04</td><td>1.2 ± 0.09</td><td>1.97 ± 0.32</td></tr><tr><td>J132633</td><td>0.32</td><td>0.12<sup>+0.14</sup><sub>-0.08</sub></td><td>4.22 ± 0.3</td><td>60.6 ± 5.2</td><td>0.13 ± 0.01</td><td>1.61 ± 0.07</td><td>0.7 ± 0.09</td><td>6.2 ± 1.58</td></tr><tr><td>J132937</td><td>0.31</td><td>&lt;0.001</td><td>1.53 ± 0.53</td><td>7.5 ± 2.6</td><td>0.06 ± 0.02</td><td>5.01 ± 0.14</td><td>3.0 ± 0.09</td><td>0.31 ± 0.03</td></tr><tr><td>J134559</td><td>0.24</td><td>&lt;0.002</td><td>1.45 ± 0.4</td><td>8.8 ± 2.4</td><td>0.05 ± 0.01</td><td>7.94 ± 0.2</td><td>1.7 ± 0.08</td><td>0.89 ± 0.1</td></tr><tr><td>J140333</td><td>0.28</td><td>0.03<sup>+0.02</sup><sub>-0.01</sub></td><td>2.28 ± 0.18</td><td>86.7 ± 9.0</td><td>0.12 ± 0.01</td><td>0.82 ± 0.06</td><td>0.7 ± 0.09</td><td>4.2 ± 1.11</td></tr><tr><td>J144010</td><td>0.3</td><td>0.01<sup>+0.0</sup><sub>-0.0</sub></td><td>8.36 ± 0.43</td><td>39.5 ± 2.2</td><td>0.14 ± 0.01</td><td>5.61 ± 0.12</td><td>0.9 ± 0.09</td><td>7.13 ± 1.44</td></tr><tr><td>J154050</td><td>0.29</td><td>&lt;0.001</td><td>3.78 ± 0.41</td><td>15.2 ± 1.7</td><td>0.09 ± 0.01</td><td>6.97 ± 0.12</td><td>1.1 ± 0.09</td><td>3.04 ± 0.48</td></tr><tr><td>J155945</td><td>0.23</td><td>&lt;0.025</td><td>4.39 ± 0.18</td><td>56.2 ± 2.7</td><td>0.27 ± 0.02</td><td>4.18 ± 0.1</td><td>0.7 ± 0.07</td><td>3.76 ± 0.92</td></tr><tr><td>J160437</td><td>0.31</td><td>&lt;0.007</td><td>2.56 ± 0.37</td><td>106.5 ± 23.0</td><td>0.07 ± 0.01</td><td>0.58 ± 0.09</td><td>0.9 ± 0.09</td><td>4.13 ± 0.85</td></tr><tr><td>J164607</td><td>0.29</td><td>0.02<sup>+0.01</sup><sub>-0.01</sub></td><td>4.82 ± 0.45</td><td>65.0 ± 7.5</td><td>0.17 ± 0.02</td><td>2.14 ± 0.14</td><td>0.7 ± 0.09</td><td>6.25 ± 1.6</td></tr><tr><td>J172010</td><td>0.29</td><td>0.03<sup>+0.03</sup><sub>-0.01</sub></td><td>3.11 ± 0.34</td><td>65.0 ± 8.6</td><td>0.22 ± 0.03</td><td>1.35 ± 0.1</td><td>1.4 ± 0.09</td><td>0.71 ± 0.12</td></tr></tbody></table>

**Note.**

a Values taken from S. R. Flury et al. (2022a).

SDSS spectra, such that

$$f_{\text{esc}}^{\text{Ly}\alpha} = \frac{F_{\text{Ly}\alpha}}{\eta(n_e, T_e) \times F_{\text{H}\beta}, \text{dustcorr}} \quad (8)$$

The values for the ratio between intrinsic Lyα and Hβ fluxes η (n_e, T_e) are calculated using the Python package PyNeb (V. Luridiana et al. 2015). This calculation uses the dust-corrected Hβ flux, n_e, and T_e measurements presented in S. R. Flury et al. (2022a).

Generally, the Lyα observables obtained via global photometry are slightly larger than the values obtained in COS spectroscopy. A few galaxies have Lyα luminosities and escape fractions lower with global photometry than the

spectroscopic value. After inspection of the spectra and images, we deduce that this might be caused by a combination of UV continuum placement in spectroscopy, potentially impacting the EW and uncertainties on the zero-point offsets in the imaging. If the spectroscopic EW is overestimated, and/or if the zero-point offset in the imaging leads to underestimating the UV continuum flux, the Lyα emission within the COS aperture in photometry would be fainter than what is measured with spectroscopy. In a compact galaxy, this would produce a global photometric Lyα flux that is lower than what is measured in COS spectra. However, due to the general agreement between the Lyα luminosity values, we determine that this is not a significant issue. Additionally, the relative
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Comparison between Lyα observables obtained via COS spectroscopy and global photometry. From left to right, the panels show comparisons for the Lyα luminosity ($L_{Lyα}$), rest-frame equivalent width (EW$_{Lyα}$), and escape fraction ($f_{esc}^{Lyα}$). The solid line shows the 1:1 relation. The global values from photometry are in broad agreement with the spectral values, indicating the galaxies are compact.</figcaption></figure>

agreement between the photometric and spectroscopic values indicates that the Lyα emission in LaCOS galaxies is compact: in most cases, the bulk of the Lyα emission is observed within the 2.5 COS aperture.

## 5. Correlations with LyC Escape

Here, we compare the global galaxy properties measured with HST photometry to $f_{esc}^{LyC}$ measured within the COS aperture. Specifically, we look at properties previously investigated via COS spectroscopy and through the COS acquisition images (S. R. Flury et al. 2022b). We use the Kendall $τ$ coefficient and associated $p$-value to assess the degree of correlation between variables, using the scheme developed in T. Isobe et al. (1986) that allows for the inclusion of censored data.<fnref n="30" /> In Table 2, we show the Kendall $τ$ and $p$-value between $f_{esc}^{LyC}$ and different galaxy properties obtained for the LaCOS sample with either COS or SBC. Following the convention in LzLCS studies, we deem a correlation ($τ > 0$) or anticorrelation ($τ < 0$) significant when $p < 0.00135$ (S. R. Flury et al. 2022b), corresponding to a $3σ$ confidence that the null hypothesis is rejected. Additionally, we describe correlations as tentative when $0.00135 < p < 0.05$.

### 5.1. Global Lyα Photometry and LyC Escape

Here, we directly show how slight changes in the Lyα observables when obtained via photometry impact correlations with the LyC escape fraction. In Figure 7, we show scatter plots comparing Lyα observables ($L_{Lyα}$, EW$_{Lyα}$, and $f_{esc}^{Lyα}$) to $f_{esc}^{LyC}$. We find a significant correlation between $f_{esc}^{LyC}$ and photometric EW$_{Lyα}$, and tentative correlations with $L_{Lyα}$ and $f_{esc}^{Lyα}$. As compared to spectroscopic values within the COS aperture, the global Lyα luminosity and escape fractions show a decreased degree of correlation with the LyC escape fraction, as probed by the Kendall $τ$ coefficient and associated $p$-values. This is sensible, as the Lyα photometric values generally probe emission on larger scales than those measured with the COS aperture, not necessarily representing the Lyα properties of the regions preferentially leaking LyC in the galaxy. The

<table><caption>**Table 2**

Strength of the Correlations between the LyC Escape Fraction and Parameters Measured Using Either COS or SBC Data</caption><thead><tr><th rowspan="2">par vs. f<sub>esc</sub></th><th colspan="2">SBC<sub>tot</sub><sup>a</sup></th><th colspan="2">SBC<sub>0.1</sub> ap<sup>b</sup></th><th colspan="2">COS<sup>c</sup></th></tr><tr><th>τ</th><th>p</th><th>τ</th><th>p</th><th>τ</th><th>p</th></tr></thead><tbody><tr><td>L<sub>Lyα</sub></td><td>0.25</td><td>1.2e-2</td><td><strong>0.44</strong></td><td><strong>1e-5</strong></td><td><strong>0.37</strong></td><td><strong>1.8e-4</strong></td></tr><tr><td>EW<sub>Lyα</sub></td><td><strong>0.36</strong></td><td><strong>2.8e-4</strong></td><td><strong>0.41</strong></td><td><strong>3.7e-5</strong></td><td><strong>0.38</strong></td><td><strong>1.1e-4</strong></td></tr><tr><td>f<sub>esc</sub><sup>Lyα</sup></td><td>0.29</td><td>3.7e-3</td><td>...</td><td>...</td><td><strong>0.43</strong></td><td><strong>1.6e-5</strong></td></tr><tr><td>r<sub>50</sub></td><td><strong>-0.35</strong></td><td><strong>3.3e-04</strong></td><td>...</td><td>...</td><td><strong>-0.38</strong></td><td><strong>9.9e-5</strong></td></tr><tr><td>Σ<sub>SFR</sub></td><td>0.27</td><td>6.1e-3</td><td>...</td><td>...</td><td>0.24</td><td>1.5e-2</td></tr></tbody></table>

**Notes.** The Kendall $τ$ and $p$-value are calculated on matched samples for the different measurements. Consistent with LzLCS studies, a correlation is deemed significant if $p < 0.00135$, consistent with previous LzLCS studies, in which case the values are shown in bold.

<sup>a</sup> Values measured in a large aperture in SBC photometry.

<sup>b</sup> Values measured in SBC photometry in a 0.1 aperture centered on the brightest UV continuum pixel.

<sup>c</sup> Values measured in the COS aperture.

global photometric EW, however, shows a slightly larger degree of correlation with the escape fraction ($τ = 0.41$, $p = 4.1 \times 10^{-5}$) as compared to the spectroscopic values ($τ = 0.38$, $p = 9.9 \times 10^{-5}$). Nevertheless, the change is relatively small and can be accounted for by the uncertainty on photometric values and general uncertainties on $τ$ for spectroscopic values (~0.05 S. R. Flury et al. 2022b). In the following, we look at the impact of sub-kiloparsec-scale Lyα properties on LyC escape.

### 5.2. Resolved Lyα Properties and LyC Escape

Few studies have been able to observe the impact of resolved galaxy properties on $f_{esc}^{LyC}$. The first study looking at the impact of resolved Lyα properties on LyC escape fractions was conducted on the Sunburst Arc, a lensed galaxy at $z \sim 2.4$ (K. J. Kim et al. 2023). In this object, LyC emission originates from a single compact star-forming region that appears to be a star cluster $r_{eff} \sim 8$ pc in size (U. Meštrić et al. 2023; T. E. Rivera-Thorsen et al. 2024). The gravitational lens has produced 12 distinct images of this region, all sampling different lines of sight to the LyC-emitting cluster. The LyC-emitting region has extreme Lyα properties compared to

<footnote n="30">³⁰ We use the code developed in E. C. Herenz et al. (2025) and adapted from (S. R. Flury et al. 2022b), publicly available on https://github.com/Knusper/kendall.</footnote>
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Global photometric Lyα observables vs. $f_{esc}^{LyC}$. From left to right, the x-axis shows the Lyα luminosity, EW, and Lyα escape fraction. Black filled circles show galaxies confidently detected in LyC, while gray arrows show galaxies with LyC escape fraction upper limits. Kendall τ and p-values assessing the degree of correlation between the x- and y-axes variables are shown on the top right corner of each panel. The escape fraction shows the same trends with photometric Lyα observables as those identified with the spectroscopic values (S. R. Flury et al. 2022b), but the correlation with Lyα luminosity and escape fraction is weaker, while $f_{esc}^{LyC}$ shows a slightly larger degree of correlation with the photometric Lyα EW.</figcaption></figure>

non-emitting regions in the rest of the galaxy and compared to the global galaxy properties. Specifically, it has the largest EW$_{Lyα}$ (~43 Å as compared to ~13 Å in non-LyC-emitting regions) and $f_{esc}^{Lyα}$ (~30%, as compared to 13% in non-LyC-emitting regions) in the galaxy (K. J. Kim et al. 2023).

For other galaxies with resolved LyC and Lyα measurements, the picture is not as simple. In nearby galaxy Haro 11, the star-forming region with the highest $f_{esc}^{Lyα}$ shows higher $f_{esc}^{LyC}$; however, the LyC luminosity is almost double in the knot with the lowest $f_{esc}^{Lyα}$ (L. Komarova et al. 2024). Additionally, Lyα and LyC observations of the LyC emitter with the highest $f_{esc}^{LyC}$ (~90%) currently known at z ~ 3.6 show weak Lyα emission in the LyC-emitting region (R. Marques-Chaves et al. 2024). This is expected in a case where nearly all of the LyC escapes, indicating a very low HI content, and thus, little material for Lyα production, on the line of sight to the starburst. With only a few LyC-emitting galaxies with sub-kiloparsec Lyα measurements, all occupying different redshift ranges, the link between resolved Lyα and LyC emission is currently not well established.

The resolution reached by LaCOS (with a PSF FWHM corresponding to ~400 pc) enables studies of the impact of resolved galaxy properties on the LyC escape fraction in a larger galaxy sample than possible before. However, resolved LyC measurements cannot currently be obtained with available instrumentation. Thus, in the following, we measure the impact of resolved properties on the line of sight $f_{esc}^{LyC}$ integrated over the COS aperture. Specifically, we investigate possible correlations between the LyC escape fraction and resolved Lyα and UV continuum observables. We also note that without Hα photometry currently available, the sub-kiloparsec Lyα escape fraction cannot be derived.

To investigate the impact of sub-kiloparsec-scale Lyα properties on $f_{esc}^{LyC}$, we specifically derive Lyα and UV continuum observables within a small aperture (3 pixels in diameter, corresponding to 0.12, roughly the PSF FWHM) centered on the brightest UV continuum sources in each galaxy. Figure 8 shows scatter plots presenting $f_{esc}^{LyC}$ as a function of $L_{Lyα}$, EW$_{Lyα}$, and $f_{λ,UV}$ obtained in this aperture, as well as $\Delta_{max}$, Lyα - cont, the spatial offset between the

brightest UV continuum and Lyα pixel. The figure also shows UV continuum, Lyα, and F850LP images of J172010 with the aperture overlaid as an example.

Inspecting the scatter plots in Figure 8, we do not find any indication of correlation between the UV continuum flux in the brightest UV continuum region and $f_{esc}^{LyC}$ ($\tau = 0.02, p = 0.8$). However, we find excellent correlations between both the $L_{Lyα}$ ($\tau = 0.44, p = 1 \times 10^{-5}$) and EW$_{Lyα}$ ($\tau = 0.41, p = 3.7 \times 10^{-5}$), and $f_{esc}^{LyC}$, as well as a tentative anticorrelation between $f_{esc}^{LyC}$ and $\Delta_{max}$, Lyα - cont ($\tau = -0.29, p = 3.7 \times 10^{-3}$). The correlation between Lyα observables and $f_{esc}^{LyC}$ is stronger when evaluated in small apertures, rather than when measured in the COS aperture or via global photometry. These results indicate that the brightest LyC-emitting regions in a galaxy may contribute significantly to the escaping LyC budget on the line of sight. Specifically, the presence of neutral gas in front of these sources would result in lower, or negative $L_{Lyα}$ and EW$_{Lyα}$, and larger offsets between the peak Lyα and UV continuum pixels. Thus, we interpret the trends observed in Figure 8 as an indication that a few compact, bright, and unobscured UV continuum sources might drive the observed LyC escape in LaCOS galaxies.

### 5.3. LyC Escape and Compactness

Small galaxy sizes and high star formation rate surface density $\Sigma_{SFR}$, have been linked to increased LyC escape fractions (T. M. Heckman et al. 2001; F. Marchi et al. 2018; R. Cen 2020; R. P. Naidu et al. 2020; J. Y. C. Yeh et al. 2023; R. Marques-Chaves et al. 2024). The star formation rate surface density is a function of the half-light radius $r_{50}$, defined as

$$ \Sigma_{SFR} = \frac{SFR}{2 \pi r_{50}^2}. \qquad (9) $$

It has been suggested as a probe of star formation feedback, a mechanism thought to be responsible for clearing the neutral ISM around ionizing sources and facilitating LyC escape (e.g., A. E. Jaskot et al. 2017; S. Gazagnes et al. 2020; L. Komarova et al. 2021; R. O. Amor\`in et al. 2024; C. A. Carr et al. 2025; S. R. Flury et al. 2025). Both decreased $r_{50}$ and increased $\Sigma_{SFR}$
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Line-of-sight LyC escape fraction measured in the COS aperture as a function of observables extracted in a 0.12 aperture centered around the brightest UV continuum pixel in each galaxy. The first top row of panels shows the position of the aperture on the UV continuum (left), Lyα (middle), and F850LP (right) images of galaxy J172010 as an example. The panels below show on their x-axis, from top to bottom and left to right, the Lyα luminosity, EW, the UV continuum flux density, and the spatial offset between the brightest UV continuum pixel and the brightest Lyα pixel as a function of $f_{\text{esc}}^{\text{LyC}}$. We show the Kendall τ and associated p-value assessing the degree of correlation between variables in the top corners of the plots. In general, there is a high degree of correlation between the Lyα observables within the aperture and the LyC escape fraction.</figcaption></figure>

have been found to correlate with $f_{\text{esc}}^{\text{LyC}}$ in the LzLCS sample (S. R. Flury et al. 2022b). However, previous measurements of the UV half-light radius for LzLCS galaxies were obtained from relatively shallow and vignetted COS acquisition images. This could potentially bias measurements and affect correlations between $r_{50}$, $\Sigma_{\text{SFR}}$, and the escape fraction, especially in the more extended galaxies.

Here, we rederive the half-light radius $r_{50}$ previously measured from COS acquisition images, using instead the deeper SBC F165LP photometry. To do so, we integrate the F165LP light profiles in increasingly larger circular apertures centered on the brightest pixel in the image. We use a 0.5 pixel increment and compute the flux as a function of radius. We determine the flux density by integrating within a circular
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Comparison between UV half-light radius calculated using COS acquisition images and SBC photometry. A solid black line indicates the 1:1 relation. The $r_{50}$ values obtained with SBC are slightly larger, but in agreement with COS measurements, except for galaxy J095700.</figcaption></figure>

aperture at which the flux reaches a plateau (less than $10^{-3}$ fractional flux difference). Then, $r_{50}$ is determined from the same profile as the radius at which the galaxy reaches half of its flux density in the F165LP image. The comparison between COS acquisition images and SBC frames $r_{50}$ is presented in Figure 9. The SBC $r_{50}$ values are systematically larger than the COS ones. In most cases, the difference is relatively small, with the SBC $r_{50}$ being larger by an average of 0.5 kpc, or $0.1$. One galaxy is a significant outlier, with an SBC $r_{50}$ that is 6 times that of the COS value. This is J095700, the largest object in the sample (see Figure 13 in Appendix C) that appears to be an actively merging system. This galaxy is large both in the rest-frame optical and UV, but the bulk of the UV emission is concentrated in a clump in the center. The fainter, extended UV emission was likely missed in the COS acquisition image since it is shallow and vignetted. However, this galaxy constitutes the main exception to the relatively small offset between $r_{50}$ derived from COS and SBC. This indicates that, as expected, the deeper and non-vignetted SBC photometry does recover light from extended regions, resulting in the galaxies being slightly larger than found in previous LzLCS studies. Nevertheless, with a characteristic median $r_{50} = 0.8$ kpc, the galaxies are still very compact.

We next look at the link between the newly derived half-light radius $r_{50}$ and the LyC escape fraction. We additionally rederive $\Sigma_{SFR}$ using the new $r_{50}$ values and the H$\beta$ star formation rate following Equation (9). Plots showing the escape fraction as a function of $r_{50}$ and $\Sigma_{SFR}$ are shown on the top and bottom panels of Figure 10, respectively. Generally, we find similar trends with the newly derived photometric values and the spectroscopic measurements. The newly obtained $\tau$ values are within the typical uncertainties on $\tau \sim 0.05$ in spectroscopy (S. R. Flury et al. 2022b; see Table 2). Thus, the improved precision on $r_{50}$ still indicates that concentrated star formation is key to LyC escape. The escaping LyC emission is likely to originate from small, compact, and bright star-forming regions that create the conditions for increased stellar feedback, rather than diffuse ones. This echoes the results on the strong correlation between sub-kiloparsec Ly$\alpha$ observables and $f_{\text{esc}}^{\text{LyC}}$.

<figure><img src="image_13.png" /><figcaption>**Figure 10.** Top panel: LyC escape fraction as a function of $r_{50}$ obtained from F165LP SBC images. Bottom panel: LyC escape fraction as a function of $\Sigma_{SFR}$. The trends with $f_{\text{esc}}^{\text{LyC}}$ are the same as those measured with spectroscopy, but the correlations are weaker.</figcaption></figure>

## 6. Summary

We have presented HST photometric data and data products for the LaCOS sample, a subset of 42 low-$z$ LyC-emitting and non-emitting galaxies representative of galaxies in the LzLCS. LaCOS observations include data obtained in three rest-frame optical bands and in two rest-frame UV bands, sampling the stellar continuum and Ly$\alpha$ line. We present optical RGB composite, Ly$\alpha$ flux, and EW maps for all galaxies in the sample. The LaCOS objects show a wide range of morphologies, but are mostly irregular galaxies, both for the LyC leakers and non-leakers. Additionally, they have a variety of Ly$\alpha$ flux and EW morphologies and distributions. Forthcoming manuscripts will specifically investigate the impact of galaxy morphology, merger interaction, and Ly$\alpha$ halo extent on LyC production and escape.

With the newly obtained HST photometry, we reinvestigate correlations previously examined with spectroscopic data between LyC escape fraction and global galaxy properties. Specifically, we rederive the Ly$\alpha$ luminosity, equivalent width, the Ly$\alpha$ escape fraction, the half-light radius, and the star formation rate surface density. We find similar trends with $f_{\text{esc}}^{\text{LyC}}$ for measurements derived through photometry as those
previously found with spectroscopic measurements. Generally, the trends with $f_{esc}^{LyC}$ show a weaker degree of correlation when obtained with global photometry rather than spectroscopy, which could be due to the uncertainties on photometric values, and to photometry recovering light from extended regions that do not contribute significantly to the escaping LyC photon output on the line of sight. With global photometry, we confirm a robust correlation ($p < 0.00135$) between $f_{esc}^{LyC}$ and $EW_{Lyα}$, a robust anticorrelation between $f_{esc}^{LyC}$ and $r_{50}$, and find tentative correlations ($p < 0.05$) between $f_{esc}^{LyC}$ and $L_{Lyα}$, $f_{esc}^{Lyα}$, and $\Sigma_{SFR}$.

Finally, we investigate possible correlations between sub-kiloparsec galaxy properties and line of sight $f_{esc}^{LyC}$ derived from the COS spectra. Specifically, we measure Lyα and UV continuum in small apertures around the brightest UV continuum source in each galaxy. While we cannot currently investigate correlations with $f_{esc}^{Lyα}$, we find strong correlations between the line of sight $f_{esc}^{LyC}$ and the Lyα luminosity and EW. The correlations with $L_{Lyα}$ and $EW_{Lyα}$ are much stronger when investigated on sub-kiloparsec scales (~400 pc) than on larger scales with photometry or spectroscopy. Additionally, we do not find any correlation between the continuum flux density around the brightest continuum source and $f_{esc}^{LyC}$, and find a tentative correlation ($p = 0.0037$) between the spatial offset between peak UV continuum and peak Lyα emitting pixels. We interpret these results as evidence that the brightest UV continuum sources in LaCOS galaxies likely contribute a large fraction of the escaping LyC radiation on the line of sight, and that the lack of neutral gas in front of these sources (as traced by smaller Lyα-to-UV offsets and higher $L_{Lyα}$ and $EW_{Lyα}$) likely plays a determining role on the line of sight $f_{esc}^{LyC}$.

## Acknowledgments

This research is based on observations made with the NASA/ESA HST obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are from HST GO programs 17069, 14131, and 11107. A.L.R. acknowledges support from HST GO-17069. M.J.H. is supported by the Swedish Research Council (Vetenskapsrådet) and is a fellow of the Knut and Alice Wallenberg Foundation. A.S.L. acknowledges support from the Knut and Alice Wallenberg Foundation. S.B. acknowledges support from NSF grant 2408050. F.L. acknowledges funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. C3UBES-101107619.

Software: NumPy (C. R. Harris et al. 2020), astropy (Astropy Collaboration et al. 2013, 2018, 2022), matplotlib (J. D. Hunter 2007), spectres (A. C. Carnall 2017), photutils (L. Bradley et al. 2024), dust_extinction (K. D. Gordon 2024), astroscrappy (P. G. van Dokkum 2001; C. McCully et al. 2018), CROCOA (A. Runnholm & J. Melinder 2024), SciPy (P. Virtanen et al. 2020), cmocean (K. M. Thyng et al. 2016), castro.

## Appendix A: Observation Parameters

In Table 3, we present the parameters for the HST observations of LaCOS galaxies. In particular, we specify the exposure time per filter for each galaxy and the temperature range of the detectors for SBC filters.
## Appendix B
### COS Aperture

Obtaining accurate Lyα maps with the method presented in this manuscript requires precisely matching the photometric aperture used to calculate the bandpass normalization factor α to the actual COS aperture. We initially consider an aperture taking into account the vignetting of the COS aperture and the mask used to extract high signal-to-noise ratio Lyα spectra in S. R. Flury et al. (2022a; hereafter, aperture 1). This extraction mask corresponds to a slit of 0.637 width oriented along the COS cross-dispersion axis. However, using aperture 1 results in significant offset Lyα fluxes between HST photometry and COS spectroscopy, with spectroscopic fluxes being systematically higher on the order of 75%. We also compare the photometric flux extracted in the COS aperture for the individual UV filters, F150LP and F165LP, to synthetic photometry obtained with the COS spectra. Depending on the filter considered, we find that fluxes obtained from the COS spectra are systematically larger by 40%–45%, which cannot be accounted for by the flux uncertainty on COS or SBC (<5%).

To assess whether the offset could be due to flux calibration issues with SBC or COS data, we compare these measurements to independent estimates of the FUV flux obtained from GALEX (shown in Figure 11), since the SBC F150LP and GALEX FUV filter overlap considerably (see bottom left panel of the figure). We compare GALEX FUV fluxes with the SBC F150LP fluxes obtained within a 6" aperture in radius. While the FUV and F150LP filter coverage are very similar, their bandpasses differ slightly. Thus, to enable comparison, we correct the SBC fluxes for the difference in bandpass coverage using the available COS spectra. Specifically, we perform synthetic photometry on the COS spectra to calculate the flux that would be measured, respectively, with the SBC F150LP and GALEX FUV filters for a given galaxy. The correction

factor applied to SBC fluxes is the ratio of the synthetic photometry fluxes, ranging from 0.89 to 1.06. We find a relatively good agreement between the GALEX and bandpass-corrected SBC fluxes, with most data points hovering around the 1:1 line, as can be seen on the top left panel of Figure 11. Therefore, we conclude that the offset between COS and SBC fluxes is not due to SBC flux calibration issues.

Next, we compare the COS synthetic photometry fluxes to those obtained with GALEX. Since the COS aperture is significantly smaller than that of GALEX (at most, a circular aperture with 1.25 radius, as opposed to a 6" radius aperture) and to make a comparison possible, we correct COS fluxes using the SBC images. To obtain correction factors, we calculate the ratio of fluxes computed within the COS and GALEX apertures in the SBC F150LP images. The correction factors range from 1.62 to 5.10. We find a strong disagreement between COS and GALEX fluxes when using aperture 1, with COS fluxes being higher than GALEX values by ~60% (see the middle panel of Figure 11).

To further investigate whether the discrepancy is caused by the assumption on shape aperture, we test the effect of using the simpler vignetted circular aperture that is 2.5 in diameter instead (hereafter aperture 2, shown on the bottom right panel of Figure 11). When repeating the analysis outlined above with this aperture, we find a better agreement between GALEX and COS fluxes, with COS fluxes being 10% lower than GALEX on average, as demonstrated in the top right panel of Figure 11. Additionally, this aperture solves the disagreement between COS and SBC F150LP and F165LP fluxes within the aperture. The fact that this aperture leads to better agreement with photometry as opposed to aperture 1 could be caused by optics smearing the target flux in the cross-dispersion direction (private communication with STScI). Thus, we choose to conduct subsequent analysis and to derive the Lyα maps using aperture 2.
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Comparison of GALEX, SBC, and COS fluxes. The top left panel shows the bandpass-corrected SBC F150LP fluxes extracted in a 6" radius aperture, similar to the GALEX one. The two other top panels show the fluxes from COS synthetic photometry, corrected using F150LP photometry (due to the small sizes of the apertures) to match the value that would be measured in the GALEX aperture. The middle top panel shows what would be measured with an aperture combining the COS vignetting profile and LzLCS spectral extraction window (aperture 1), and the right top panel, the circular 2.5" diameter aperture value (aperture 2). Blue lines show linear fits to the data. The bottom panels show, from left to right, the normalized bandpass response of GALEX FUV and SBC F150LP overlaid on the spectrum for J172010 spectra, and the two apertures used to correct the COS synthetic FUV photometry, overlaid on the Lyα image of J172010.</figcaption></figure>

## Appendix C: Comparison to LaXs

In the low-z Universe ($z \sim 0.03$), the pixel-wise SED fitting software LaXs (M. Hayes et al. 2009) has been commonly used to obtain $Ly\alpha$ maps of star-forming galaxies (M. Hayes et al. 2014; J. Melinder et al. 2023). Most galaxies in LaCOS do not have the number of filters required for robust pixel-by-pixel fitting of the data using LaXs. However, two galaxies in the sample were observed as part of archival HST programs containing additional filters. In particular, J12483 was observed as part of HST program 14131, and J092532 as part of program 14466, both with additional ACS/WFC observations in filter FR853N covering the $H\alpha$ line. This photometric coverage allows for robust pixel-by-pixel fitting of the data, allowing for $Ly\alpha$ maps to be produced with LaXs. In this section, we compare the $Ly\alpha$ and UV continuum maps obtained for J124835 and J092532 using the method devised for the LaCOS sample with those obtained from photometric fitting using LaXs. To rule out any possible differences coming from different data reduction methods, we apply the methods used for the LaCOS sample to the science frames reduced by collaborators for J124835 and J092532 and used in the LaXs run (described in detail in J. Melinder et al. 2025, in preparation).

The maps showing LaCOS and LaXs $Ly\alpha$ and UV continuum maps for J092532 and J124835 are shown in Figure 12. The UV continuum maps are extremely similar for both derivations, showing the same morphology but with slightly brighter emission in LaXs maps. However, the LaXs

$Ly\alpha$ maps are much fainter than the LaCOS ones, with a similar global morphology, but with additional $Ly\alpha$ absorption regions in LaXs. Table 4 presents the $Ly\alpha$ flux, continuum flux density, and $Ly\alpha$ EW values integrated within the COS aperture for the two galaxies using maps produced. By construct, the equivalent width within the COS aperture is the same for the LaCOS map and the COS spectrum. However, the LaXs-derived EW value in the COS aperture is smaller by a significant amount. The UV continuum flux density values are smaller (with an average 17% relative difference) for LaCOS as compared to the LaXs map. The $Ly\alpha$ flux values in the photometric maps are smaller than those derived from COS spectroscopy; however, the LaXs value is smaller by a larger amount than the LaCOS maps (with an average 34% relative difference between LaCOS and LaXs). This indicates that, at least within the COS extraction aperture, the LaCOS maps are in better agreement with the values from spectroscopy, which is expected given the method employed to derive the maps.

The $Ly\alpha$ measurements from the maps derived using LaXs are significantly different from the spectroscopic values. This behavior has previously been noted with LaXs, and may be due to continuum emission being overestimated by the software in regions with low signal to noise. It is beyond the scope of this study to investigate and correct for this offset, but possible causes could be uncertainties in the absolute calibration of HST instruments or assumptions underpinning calculations in LaXs. However, we note this could impact comparison of $Ly\alpha$ observables in high-z studies and those in the very nearby Universe, where LaXs has been employed (e.g., J. Melinder et al. 2023).
<figure><img src="image_15.png" /><figcaption>**Figure 12.** Comparison between LaCOS (top panels) and LaXs (bottom panels) Lyα and UV continuum maps for J092532 (left set of panels) and J124835 (right set of panels).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 13.** Optical RGB composite (top panels) and Lyα images (bottom panels) for galaxies with optical extent exceeding 20 kpc on the side. The data are the same as shown in Figures 3 and 4, but on scales that showcase the emission visible across all filters. The white bar at the bottom of the RGB composite panels indicates the 20 kpc scale. These galaxies all have very tenuous Lyα emission, which is the reason why it is barely visible in the lower panels.</figcaption></figure>
<table><caption>**Table 3**
List of Exposure Times and Detector Temperatures</caption><thead><tr><th>ID</th><th>t<sub>F150LP</sub><br>(s)</th><th>t<sub>F165LP</sub><br>(s)</th><th>t<sub>F438W</sub><br>(s)</th><th>t<sub>F547M</sub><br>(s)</th><th>t<sub>F850LP</sub><br>(s)</th><th>T<sub>F150LP</sub><br>(C)</th><th>T<sub>F165LP</sub><br>(C)</th></tr></thead><tbody><tr><td>J011309</td><td>2000</td><td>2568</td><td>508</td><td>613</td><td>696</td><td>17.9-25.1</td><td>18.9-24.8</td></tr><tr><td>J012910</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>18.7-25.6</td><td>19.6-25.3</td></tr><tr><td>J072326</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>18.7-25.5</td><td>19.6-25.1</td></tr><tr><td>J081409</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>18.9-26.1</td><td>20.1-25.8</td></tr><tr><td>J082652</td><td>1984</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>17.7-25.6</td><td>18.8-25.3</td></tr><tr><td>J090918</td><td>1980</td><td>1947</td><td>508</td><td>622</td><td>696</td><td>18.4-25.9</td><td>19.4-25.5</td></tr><tr><td>J091113</td><td>1000</td><td>1311</td><td>508</td><td>624</td><td>696</td><td>24.8-26.0</td><td>25.3-25.6</td></tr><tr><td>J091207</td><td>2000</td><td>2616</td><td>508</td><td>637</td><td>696</td><td>18.2-26.6</td><td>19.2-26.2</td></tr><tr><td>J091703</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>17.7-24.7</td><td>18.7-24.4</td></tr><tr><td>J092532</td><td>2244</td><td>6133</td><td>900</td><td>1499</td><td>776</td><td>17.8-25.5</td><td>19.9-24.7</td></tr><tr><td>J092552</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>18.2-25.4</td><td>19.4-24.9</td></tr><tr><td>J093355</td><td>1962</td><td>2610</td><td>508</td><td>637</td><td>696</td><td>18.2-25.1</td><td>19.2-24.7</td></tr><tr><td>J095236</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>19.0-26.2</td><td>20.1-25.9</td></tr><tr><td>J095700</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>17.4-24.7</td><td>18.4-24.3</td></tr><tr><td>J095838</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>18.2-25.4</td><td>19.2-25.0</td></tr><tr><td>J105331</td><td>2000</td><td>2616</td><td>508</td><td>637</td><td>696</td><td>18.9-26.1</td><td>19.9-25.8</td></tr><tr><td>J110452</td><td>2000</td><td>2598</td><td>508</td><td>628</td><td>696</td><td>18.2-25.4</td><td>19.4-25.0</td></tr><tr><td>J112224</td><td>1485</td><td>1919</td><td>508</td><td>624</td><td>696</td><td>18.2-23.8</td><td>19.2-24.3</td></tr><tr><td>J113304</td><td>2715</td><td>2323</td><td>508</td><td>665</td><td>696</td><td>13.8-16.3</td><td>18.7-21.5</td></tr><tr><td>J115855</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>20.6-27.6</td><td>21.6-27.3</td></tr><tr><td>J115959</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>18.4-25.6</td><td>19.5-25.3</td></tr><tr><td>J120934</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>18.9-25.9</td><td>19.9-25.6</td></tr><tr><td>J121915</td><td>2000</td><td>2598</td><td>508</td><td>628</td><td>696</td><td>27.1-29.3</td><td>27.3-29.2</td></tr><tr><td>J124033</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>17.9-24.9</td><td>19.2-24.5</td></tr><tr><td>J124423</td><td>2000</td><td>2568</td><td>508</td><td>613</td><td>696</td><td>18.4-25.6</td><td>19.4-25.3</td></tr><tr><td>J124835</td><td>3080</td><td>8160</td><td>900</td><td>1499</td><td>764</td><td>17.3-24.5</td><td>19.4-24.7</td></tr><tr><td>J125503</td><td>2000</td><td>2588</td><td>508</td><td>623</td><td>696</td><td>18.5-25.6</td><td>19.6-25.3</td></tr><tr><td>J125718</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>18.9-25.9</td><td>19.9-25.6</td></tr><tr><td>J130559</td><td>1978</td><td>2568</td><td>508</td><td>624</td><td>696</td><td>18.1-24.9</td><td>19.2-24.5</td></tr><tr><td>J131037</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>18.7-25.6</td><td>19.6-25.3</td></tr><tr><td>J131419</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>18.4-25.6</td><td>19.6-25.3</td></tr><tr><td>J131904</td><td>1984</td><td>2584</td><td>508</td><td>637</td><td>696</td><td>18.9-21.8</td><td>19.9-21.1</td></tr><tr><td>J132633</td><td>1984</td><td>2558</td><td>508</td><td>624</td><td>696</td><td>18.2-21.1</td><td>19.4-20.5</td></tr><tr><td>J132937</td><td>2000</td><td>2632</td><td>508</td><td>645</td><td>696</td><td>17.9-25.0</td><td>19.2-24.7</td></tr><tr><td>J134559</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>18.7-25.9</td><td>19.9-25.6</td></tr><tr><td>J140333</td><td>2000</td><td>2648</td><td>508</td><td>653</td><td>696</td><td>18.4-25.5</td><td>19.4-25.1</td></tr><tr><td>J144010</td><td>2000</td><td>2598</td><td>508</td><td>628</td><td>696</td><td>18.4-25.5</td><td>19.4-25.1</td></tr><tr><td>J154050</td><td>2000</td><td>2632</td><td>508</td><td>645</td><td>696</td><td>17.8-24.2</td><td>18.9-23.8</td></tr><tr><td>J155945</td><td>2000</td><td>2590</td><td>508</td><td>624</td><td>696</td><td>18.8-25.6</td><td>19.9-25.4</td></tr><tr><td>J160437</td><td>2000</td><td>2576</td><td>508</td><td>617</td><td>696</td><td>18.4-25.6</td><td>19.6-25.3</td></tr><tr><td>J164607</td><td>2000</td><td>2586</td><td>508</td><td>622</td><td>696</td><td>18.4-27.0</td><td>19.6-26.8</td></tr><tr><td>J172010</td><td>2000</td><td>2616</td><td>508</td><td>637</td><td>696</td><td>19.2-26.1</td><td>20.1-25.9</td></tr></tbody></table>

Note. The exposure times are given for the different HST filters used to observe the LaCOS galaxies. The detector temperature ranges are shown for the two ACS/SBC filters, F150LP and F165LP.

<table><caption>**Table 4**
Comparison between Lyα and UV Continuum Measurements Obtained in the COS Aperture for J092532 and J124835, Using the Maps Derived Using the LaCOS Method and LaXs, and the Values Calculated from the COS Spectrum</caption><thead><tr><th rowspan="2"></th><th colspan="3">J092538</th><th colspan="3">J124835</th></tr><tr><th>F<sub>Lyα</sub><br>(10<sup>-14</sup> erg s<sup>-1</sup> cm<sup>-2</sup>)</th><th>f<sub>cont</sub><br>(10<sup>-16</sup> erg s<sup>-1</sup> cm<sup>-2</sup> Å<sup>-1</sup>)</th><th>EW<br>(Å)</th><th>F<sub>Lyα</sub><br>(10<sup>-14</sup> erg s<sup>-1</sup> cm<sup>-2</sup>)</th><th>f<sub>cont</sub><br>(10<sup>-16</sup> erg s<sup>-1</sup> cm<sup>-2</sup> Å<sup>-1</sup>)</th><th>EW<br>(Å)</th></tr></thead><tbody><tr><td>LaCOS</td><td>1.48 ± 0.04</td><td>1.53 ± 0.16</td><td>75 ± 8</td><td>3.89 ± 0.05</td><td>3.16 ± 0.26</td><td>97 ± 8</td></tr><tr><td>LaXs</td><td>1.13 ± 0.04</td><td>1.93 ± 0.02</td><td>45 ± 2</td><td>2.17 ± 0.03</td><td>3.42 ± 0.02</td><td>50 ± 1</td></tr><tr><td>COS</td><td>2.20 ± 0.09</td><td>...</td><td>75 ± 6</td><td>5.78 ± 0.14</td><td>...</td><td>97 ± 8</td></tr></tbody></table>
## Appendix D
Figures for Large Galaxies

Some of the galaxies in LaCOS have emission on scales extending beyond the 20 kpc boxes presented in Figures 3 and 4. Larger cutouts for these galaxies are presented in Figure 13.

### ORCID iDs

Alexandra Le Reste https://orcid.org/0000-0003-1767-6421

Claudia Scarlata https://orcid.org/0000-0002-9136-8876

Matthew J. Hayes https://orcid.org/0000-0001-8587-218X

Jens Melinder https://orcid.org/0000-0003-0470-8754

Alberto Saldana-Lopez https://orcid.org/0000-0001-8419-3062

Aaron Smith https://orcid.org/0000-0002-2838-9033

Axel Runnholm https://orcid.org/0000-0002-1025-7569

Yu-Heng Lin https://orcid.org/0000-0001-8792-3091

Ricardo O. Amorín https://orcid.org/0000-0001-5758-1000

Hakim Atek https://orcid.org/0000-0002-7570-0824

Sanchayeeta Borthakur https://orcid.org/0000-0002-2724-8298

Cody A. Carr https://orcid.org/0000-0003-4166-2855

Brian Fleming https://orcid.org/0000-0002-2129-0292

Sophia R. Flury https://orcid.org/0000-0002-0159-2613

Mauro Giavalisco https://orcid.org/0000-0002-7831-8751

Alaina Henry https://orcid.org/0000-0002-6586-4446

Anne E. Jaskot https://orcid.org/0000-0002-6790-5125

Zhiyuan Ji https://orcid.org/0000-0001-7673-2257

Intae Jung https://orcid.org/0000-0003-1187-4240

Floriane Leclercq https://orcid.org/0000-0002-6085-5073

Rui Marques-Chaves https://orcid.org/0000-0001-8442-1846

Stephan R. McCandliss https://orcid.org/0000-0003-0503-4667

M. S. Oey https://orcid.org/0000-0002-5808-1320

Göran Östlin https://orcid.org/0000-0002-3005-1349

Swara Ravindranath https://orcid.org/0000-0002-5269-6527

Daniel Schaerer https://orcid.org/0000-0001-7144-7182

Trinh X. Thuan https://orcid.org/0000-0001-5331-2030

Xinfeng Xu https://orcid.org/0000-0002-9217-7051

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