The sizes of bright Lyman-break galaxies at <i>z</i> ≃ 3–5 with <i>JWST</i> PRIMER

ABSTRACT We use data from the JWST Public Release IMaging for Extragalactic Research survey to measure the size scaling relations of 1668 rest-frame UV-bright Lyman-break galaxies (LBGs) at $z=3\!-\!5$ with stellar masses $\mathrm{log}_{10}({\it M}_{\star}/{\rm M}_{\odot }) \gt 9$. The sample was selected from seeing-dominated ground-based data, presenting an unbiased sampling of the morphology and size distributions of luminous sources. We fit Sérsic profiles to eight NIRCam bands and also measure a non-parametric half-light radius. We find that the size distributions with both measurements are well-fit by a lognormal distribution at all redshifts, consistent with disc formation models where size is governed by host dark-matter halo angular momentum. We find a size–redshift evolution of $R_{\rm e} = 3.51(1+z)^{-0.60\pm 0.22}$ kpc, in agreement with JWST studies. When considering the typical (modal) size over $z=3\!-\!5$, we find little evolution with bright LBGs remaining compact at $R_{\rm e}\simeq 0.7{-}0.9$ kpc. Simultaneously, we find evidence for a build-up of large ($R_{\rm e} \gt 2$ kpc) galaxies by $z=3$. We find some evidence for a negatively sloped size–mass relation at $z=5$ when Sérsic profiles are used to fit the data in F200W. The intrinsic scatter in our size–mass relations increases at higher redshifts. Additionally, measurements probing the rest-UV (F200W) show larger intrinsic scatter than those probing the rest-optical (F356W). Finally, we leverage rest-UV and rest-optical photometry to show that discy galaxies are well established by $z=5$, but are beginning to undergo dissipative processes, such as mergers, by $z=3$. The agreement of our size–mass and size–luminosity relations with simulations provides tentative evidence for centrally concentrated star formation at high redshift.