COSMOS-Web: Unraveling the Evolution of Galaxy Size and Related Properties at 2 &lt; <i>z</i> &lt; 10

Abstract We measure galaxy sizes from 2 < z < 10 using COSMOS-Web, the largest-area JWST imaging survey to date, covering ∼0.54 deg 2 . We analyze the rest-frame optical (∼5000 Å) size evolution and its scaling relation with stellar mass ( R e ∝ M * α ) for star-forming and quiescent galaxies. For star-forming galaxies, the slope α remains approximately 0.20 at 2 < z < 8, showing no significant evolution over this redshift range. At higher redshifts, the slopes are −0.13 ± 0.15 and 0.37 ± 0.36 for 8 < z < 9 and 9 < z < 10, respectively. At fixed galaxy mass, the size evolution for star-forming galaxies follows R e ∝ (1 + z ) −1.21±0.05 . For quiescent galaxies, the size–mass relation is steeper with α ∼ 0.5–0.8 at 2 < z < 5, and size evolves as R e ∝ (1 + z ) −0.81±0.26 . We find that the size−mass relation is consistent between UV and optical at z < 8 for star-forming galaxies. However, we observe a decrease in the slope from UV to optical at z > 8, with a tentative negative slope in the optical at 8 < z < 9, suggesting a complex interplay between intrinsic galaxy properties and observational effects such as dust attenuation. We discuss the ratio between galaxies’ half-light radius and underlying halos’ virial radius, R vir , and find the median value of R e / R vir = 2.7%. The star formation rate surface density evolves as log ( Σ SFR / ( M ⊙ / yr / kpc 2 ) ) = (0.20 ± 0.08) z + (−0.65 ± 0.51), and the Σ SFR – M * relation remains flat at 2 < z < 10. Lastly, we identify a threshold in stellar mass surface density log ( Σ e / ( M ⊙ / kpc 2 ) ) ∼ 9.5 − 10 marking the transition to compact, quenched galaxies from extended, star-forming progenitors. In summary, our findings show that the extensive COSMOS-Web dataset at z > 3 provides new insights into galaxy size and related properties in the rest-frame optical.