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

Abstract

We present morphologies of galaxies at $z\gtrsim 9$ resolved by JWST/NIRCam $2$-$5\mu$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 $6$ bright galaxies with S/N $=10$-$40$ 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 effective radii $r_{\mathrm{e}}$ of our measurements are consistent with those of previous measurements at $z\sim 9$. We obtain $r_{\mathrm{e}}\simeq 200$-$300$ pc with the exponential-like profiles, Sérsic indexes of $n\simeq 1$-$1.5$, for galaxies at $z\sim 12$-$16$, indicating that the relation of $r_{\mathrm{e}}\propto (1+z{)}^s$ for $s=-1.22_{-0.16}^{+0.17}$ explains cosmic evolution over $z\sim 0$-$16$ for $\sim L_{z=3}^{\ast }$ galaxies. One bright ($M_{\mathrm{UV}}=-21$ mag) galaxy at $z\sim 12$, GL-z12-1, has an extremely compact profile with $r_{\mathrm{e}}=39\pm 11$ pc that is surely extended over the PSF. Even in the case that the GL-z12-1 SB is fit by AGN$+$galaxy composite profiles, the best-fit galaxy component is again compact, $r_{\mathrm{e}}=48_{-15}^{+38}$ pc that is significantly ($>5\sigma$) smaller than the typical $r_{\mathrm{e}}$ value at $z\sim 12$. Comparing with numerical simulations, we find that such a compact galaxy naturally forms at $z\gtrsim 10$, and that frequent mergers at the early epoch produce more extended galaxies following the $r_{\mathrm{e}}\propto (1+z{)}^s$ relation.

29 pages, 25 figures, accepted for publication in ApJ

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