Early Results from GLASS-JWST. V: The First Rest-frame Optical Size-Luminosity Relation of Galaxies at z > 7

Abstract

We present the first rest-frame optical size-luminosity relation of galaxies at z > 7, using the NIRCam imaging data obtained by the GLASS James Webb Space Telescope Early Release Science (GLASS-JWST-ERS) program, providing the deepest extragalactic data of the ERS campaign. Our sample consists of 19 photometrically selected bright galaxies with m <SUB>F444W</SUB> ≤ 27.8 at 7 < z < 9 and m <SUB>F444W</SUB> < 28.2 at z ~ 9-15. We measure the size of the galaxies in five bands, from rest-frame optical (~4800 Å) to the UV (~1600 Å) based on the Sérsic model, and analyse the size-luminosity relation as a function of wavelength. Remarkably, the data quality of the NIRCam imaging is sufficient to probe the half-light radius r <SUB> e </SUB> down to ~100 pc at z > 7. Given the limited sample size and magnitude range, we first fix the slope to that observed for larger samples in rest-frame UV using Hubble Space Telescope samples. The median size r <SUB>0</SUB> at the reference luminosity M = -21 decreases slightly from rest-frame optical (600 ± 80 pc) to UV (450 ± 130 pc). We then refit the size-luminosity relation allowing the slope to vary. The slope is consistent with β ~ 0.2 for all bands except F150W, where we find a marginally steeper slope of β = 0.53 ± 0.15. The steep UV slope is mainly driven by the smallest and faintest galaxies. If confirmed by larger samples, it implies that the UV size-luminosity relation breaks toward the faint end, as suggested by lensing studies.