The Size–Mass Relation at Rest-frame 1.5 μm 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. Sérsic 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 _⋆ = 10^9.5 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 (R _e ∝ (1 + z)^‑0.7) for L* star-forming galaxies and faster evolution (R _e ∝ (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 the rest-frame near-IR with JWST, a better proxy of the underlying stellar mass distribution.