The Size-Mass relation at Rest-Frame 1.5μm from JWST/NIRCam in the COSMOS-WEB and PRIMER-COSMOS fields

We present the galaxy stellar mass - size relation in the rest-frame near-IR (1.5~μm) and its evolution with redshift up to $z=2.5$. S\'ersic profiles are measured for ~ $26\,000$ galaxies with stellar masses M_ > 10⁹~M_ 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_ = 109.5~M_ to -0.25~dex at M_ > 10¹¹~M_. 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 (Rₑ ∝ (1+z)-0.7) for L^*~star-forming galaxies and faster evolution (Rₑ ∝ (1+z)-1.3) for L^*~quiescent galaxies. Massive (M_>10¹¹~M_) star-forming galaxies evolve in size almost as fast as quiescent galaxies. Low-mass (M_<10¹⁰~M_)~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.