Morphologies of Galaxies at z ≳ 9 Uncovered by JWST/NIRCam Imaging: Cosmic Size Evolution and an Identification of an Extremely Compact Bright Galaxy at z ∼ 12

Abstract

We present morphologies of galaxies at z ≳ 9 resolved by JWST/NIRCam 2–5 μ m imaging. Our sample consists of 22 galaxy candidates identified by stringent dropout and photo- z criteria in GLASS, CEERS, SMACS J0723, and Stephan’s Quintet flanking fields, one of which has been spectroscopically identified at z = 11.44. We perform surface brightness (SB) profile fitting with GALFIT for six bright galaxies with a signal-to-noise ratio = 10–40 on an individual basis and for stacked faint galaxies with secure point-spread functions (PSFs) of the NIRCam real data, carefully evaluating systematics by Monte Carlo simulations. We compare our results with those of previous JWST studies, and confirm that the effective radii r _e of our measurements are consistent with those of previous measurements at z ∼ 9. We obtain r _e ≃ 200–300 pc with the exponential-like profiles, Sérsic indexes of n ≃ 1–1.5, for galaxies at z ∼ 12–16, indicating that the relation of r _e ∝ (1 + z ) ^s for $s=-{1.22}_{-0.16}^{+0.17}$ explains cosmic evolution over z ∼ 0–16 for $\sim {L}_{z=3}^{* }$ galaxies. One bright ( M _UV = −21 mag) galaxy at z ∼ 12, GL-z12-1, has an extremely compact profile with r _e = 39 ± 11 pc that is surely extended over the PSF. Even in the case that the GL-z12-1 SB is fit by active galactic nuclei + galaxy composite profiles, the best-fit galaxy component is again compact, ${r}_{{\rm{e}}}={48}_{-15}^{+38}$ pc, which is significantly (>5 σ ) smaller than the typical r _e value at z ∼ 12. Compared with numerical simulations, we find that such a compact galaxy naturally forms at z ≳ 10, and that frequent mergers at the early epoch produce more extended galaxies following the r _e ∝ (1 + z ) ^s relation.

Keywords

• Galactic and extragalactic astronomy
• Galaxies
• Galaxy radii
• Galaxy structure
• Galaxy formation
• Galaxy evolution