JWST UNCOVERs the Optical Size-Stellar Mass Relation at 4 < z < 8: Rapid Growth in the Sizes of Low-mass Galaxies in the First Billion Years of the Universe

We study the rest-frame optical and ultraviolet (UV) morphology of galaxies in the first billion years of the Universe. Using James Webb Space Telescope data from the UNCOVER and MegaScience surveys targeting the lensing cluster A2744, we present multiband morphological measurements for a sample of 995 galaxies selected using 20-band NIRCam photometry and 35 using NIRSpec Prism spectroscopy over the redshift range of 4 < z < 8. The wavelength-dependent morphology is measured using pysersic by simultaneously modeling the images in six NIRCam wide filters covering the rest-frame UV to optical. The joint modeling technique increases the precision of measured radii by 50%. Galaxies in our sample show a wide range of Sérsic indices, with no systematic difference between optical and UV morphology. We model the size-mass relation in a Bayesian manner using a continuity model to directly fit the redshift evolution while accounting for observational uncertainties. We find the average size of galaxies at log M * / M ⊙ = 8.5 grows rapidly, from 400 pc at z = 8 to 830 pc at z = 4. This is faster evolution than expected from power-law scalings of the Hubble parameter or scale factor that describe well previous results at z < 2. This suggests that different and/or much stronger processes affect low-mass systems during the epoch of reionization. The measured logarithmic slope (0.25) and scatter (0.23 dex) are nonevolving. We discuss the remarkable consistency of the slope and scatter over cosmic time in the context of the galaxy-halo connection.