MNRAS 000, 1–9 (2021) Preprint 1 December 2021 Compiled using MNRAS L A T E X style file v3.0 RELICS: Small Lensed  ≥ 5. 5 Galaxies Selected as Potential Lyman Continuum Leakers Chloe Neufeld, 1 Victoria Strait, 2,1 Maruša Bradač, 1 Brian C. Lemaux, 3,1 Dan Coe, 4 Lilan Yang, 5,6 Tommaso Treu, 6 Adi Zitrin, 7 Mario Nonino, 8 Larry Bradley, 4 Keren Sharon 9 1 Physics and Astronomy Department, University of California, Davis, CA 95616, USA 2 Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Denmark 3 Gemini Observatory, NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, Hawai’i, 96720, USA 4 Space Telescope Science Institute, Baltimore, MD 21218, USA 5 School of Physics and Technology, Wuhan University, Wuhan 430072, China 6 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA 7 Department of Physics, Ben-Gurion University, Be’er-Sheva 84105, Israel 8 INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, I-34131 Trieste, Italy 9 Department of Astronomy, University of Michigan, 1085 South University Ave, Ann Arbor, MI 48109, USA Accepted XXX. Received YYY; in original form ZZZ ABSTRACT We present size measurements of 78 high-redshift ( ≥ 5.5) galaxy candidates from the Reionisation Lensing Cluster Survey (RELICS). These distant galaxies are well-resolved due to the gravitational lensing power of foreground galaxy clusters, imaged by the Hubble Space Telescope (HST ) and the Spitzer Space Telescope. We compute sizes using the forward-modeling code Lenstruction and account for magnification using public lens models. The resulting size-magnitude measurements confirm the existence of many small (  eff < 200 pc) galaxies in the early universe, in agreement with previous studies. In addition, we highlight compact and highly star-forming sources as possible Lyman continuum leaker candidates with high inferred [OIII]+H that could be major contributors to the process of reionisation. Future spectroscopic follow-up of these compact galaxies (e.g., with the James Webb Space Telescope) will further clarify their role in reionisation and the physics of early star formation. Key words: galaxies: high-redshift – gravitational lensing: strong – galaxies: evolution – galaxies: fundamental parameters 1 INTRODUCTION The study of high-redshift galaxies is a key aspect of constructing a complete picture of galaxy formation and evolution. Their properties, such as size and magnitude, can lend insight into conditions in the galaxies themselves and between galaxies in the intergalactic medium (IGM) during the Epoch of Reionisation, which marks an important phase change in the universe when neutral gas was ionised by the first sources of light. By studying these early galaxies’ properties, we can address questions involving which specific objects were responsible for reionisation and how the process occurred over time. Knowledge of the size evolution of galaxies allows deeper under- standing of galaxy evolution and formation. Previous works have studied the relation between size and luminosity (Huang et al. 2013; Ono et al. 2013; Holwerda et al. 2015; Shibuya et al. 2015, 2019) as well as size and stellar mass (Franx et al. 2008; van der Wel et al. 2014; Morishita et al. 2014; Mowla et al. 2019). These relations in- volve implications for the UV Luminosity Functions (LFs), especially the faint end slope (Grazian et al. 2011; Bouwens et al. 2021), as ex- tremely small sizes at high redshifts imply a steep size-luminosity relation and a shallow faint-end slope for the LF (Kawamata et al. 2018). The faint end slope of the LF and determining the relation of galaxy sizes to their ionising photon production in the early universe are important aspects for understanding the sources of reionisation (Grazian et al. 2012), since balancing the ionising photon budgets requires sources beyond the current observational limits (Atek et al. 2015; Kawamata et al. 2015; Livermore et al. 2017). In addition, Huang et al. (2017) find that the sizes of galaxies are proportional to the sizes of their dark matter halos, and it has been shown that size decreases at higher redshifts for fixed luminosity or stellar mass (Ono et al. 2013; van der Wel et al. 2014; Morishita et al. 2014; Shibuya et al. 2015, 2019; Mowla et al. 2019), although see Ribeiro et al. (2016) for an alternate view. Sizes of galaxies have also been used to select for Lyman con- tinuum leaking galaxies, likely major contributors to the process of reionisation. As found by Marchi et al. (2018), galaxies at redshift 3.5 ≤  ≤ 4.3 that are compact, with  eff ≤ 0.3 kpc, tend to have higher inferred Lyman continuum flux (i.e., flux at < 912Å equiv- alent width) than more extended sources as well as strong Lyman- emission. Others have also found that Lyman continuum leakers are compact and highly star forming (e.g., Atek et al. 2015; Kawamata et al. 2015; Izotov et al. 2016; Livermore et al. 2017; Naidu et al. 2020; Kim et al. 2021, although see Saxena et al. 2021), and thus measuring the sizes of galaxies in the Epoch of Reionisation is key to identifying likely leakers. In this work, we study the sizes of a sample of 78 star-forming galaxies at  ≥ 5.5 using the Reionisation Lensing Cluster Sur- vey (RELICS, PI Coe) and companion survey Spitzer-RELICS (S- RELICS, PI Bradač), which provide imaging data of lensed high- © 2021 The Authors arXiv:2111.14882v1 [astro-ph.GA] 29 Nov 2021