The Size-Mass relation at Rest-Frame $1.5\hat{I}¼$m from JWST/NIRCam in the COSMOS-WEB and PRIMER-COSMOS fields

Abstract

We present the galaxy stellar mass - size relation in the rest-frame near-IR ($1.5\hat{I}¼\text{m}$) and its evolution with redshift up to $z=2.5$. Sérsic profiles are measured for $\sim$ $26000$ galaxies with stellar masses $M_{\star }>10^9{\text{M}}_{\odot }$ from JWST/NIRCam F277W and F444W imaging provided by the COSMOS-WEB and PRIMER surveys, using coordinates, redshifts, colors and stellar mass estimates from the COSMOS2020 catalog. The new rest-frame near-IR effective radii are generally smaller than previously measured rest-frame optical sizes, on average by 0.14~dex, with no significant dependence on redshift. For quiescent galaxies this size offset does not depend on stellar mass, but for star-forming galaxies the offset increases from -0.1~dex at $M_{\star }=10^{9.5}{\text{M}}_{\odot }$ to -0.25~dex at $M_{\star }>10^{11}{\text{M}}_{\odot }$. That is, we find that the near-IR stellar mass - size relation for star-forming galaxies is flatter in the rest-frame near-IR than in the rest-frame optical at all redshifts $0.5<z<2.5$. The general pace of size evolution is the same in the near-IR as previously demonstrated in the optical, with slower evolution ($R_{\text{e}}\propto (1+z{)}^{-0.7}$) for $L^{\ast }$~star-forming galaxies and faster evolution ($R_{\text{e}}\propto (1+z{)}^{-1.3}$) for $L^{\ast }$~quiescent galaxies. Massive ($M_{\star }>10^{11}{\text{M}}_{\odot }$) star-forming galaxies evolve in size almost as fast as quiescent galaxies. Low-mass ($M_{\star }<10^{10}{\text{M}}_{\odot }$)~quiescent galaxies evolve as slow as star-forming galaxies. Our main conclusion is that the size evolution narrative as it has emerged over the past two decades does not radically change when accessing with JWST the rest-frame near-IR, a better proxy of the underlying stellar mass distribution.

Accepted for publication. V2: includes a new table (table 1) of the data produced and available in machine-readable format on the journal version of the article