James Webb Space Telescope Feed Post

Harvard ADS: The Size-Mass relation at Rest-Frame 1.5\mum from JWST/NIRCam in the COSMOS-WEB and PRIMER-COSMOS fields

Paper abstract: We present the galaxy stellar mass - size relation in the rest-frame near-IR (1.5~\mu{\text{m}}) and its evolution with redshift up to z=2.5. S\'ersic profiles are measured for ~ 26\,000 galaxies with stellar masses M_* > 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_* = 10^{9.5}~{\text{M}}_\odot to -0.25~dex at M_* > 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^*~star-forming galaxies and faster evolution (R_{\text{e}} \propto (1+z)^{-1.3}) for L^*~quiescent galaxies. Massive (M_*>10^{11}~{\text{M}}_\odot) star-forming galaxies evolve in size almost as fast as quiescent galaxies. Low-mass (M_*<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.