The sizes of bright Lyman-break galaxies at z 3–5 with JWST PRIMER

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 log₁₀(M/M) > 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_e = 3.51(1 + z)^−0.60±^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_e 0.7−0.9 kpc. Simultaneously, we find evidence for a build-up of large (R_e > 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.