Star formation quenching precedes morphological transformation in COSMOS-WEB's richest galaxy groups

Abstract

We analyzed the 25 richest galaxy groups in COSMOS-Web at z = 0.18-3.65, identified via the AMICO algorithm. These groups contain 20-30 galaxies with high (>75%) membership probability. Our study reveals both passive-density and active-density relations: late-type galaxies (LTGs) prefer higher central overdensities than early-type galaxies (ETGs) across all groups, and many massive LTGs exhibit colors typical of quiescent galaxies. We identify red sequences (RS) in 5 groups, prominently established at z < 1, with early emergence in the RS locus up to z ~ 2.2. This suggests group environments represent a transitional phase where star formation quenching precedes morphological transformation, contrasting with the classical morphology-density relation in rich clusters. In the central regions (~33 arcsec / 100 kpc from centers), we identified 86 galaxies: 23 (~27%) ETGs and 63 (~73%) LTGs. High-mass galaxies (M_star > 10^10.5 M_sun) undergo rapid quenching over ~1 Gyr, becoming predominantly spheroidal ETGs. This indicates morphological transformation accelerates in massive systems during peak cosmic star formation. Intermediate-mass galaxies (10^9 < M_star/M_sun < 10^10.5) show mild quenching, while low-mass galaxies (M_star < 10^9 M_sun) remain largely star-forming; here, environmental processes suppress star formation without destroying disks, suggesting group quenching operates on longer timescales than mass quenching. Overall, mass-dependent quenching dominates the high-mass end, while environment shapes lower-mass systems. The HLAGN fraction for both groups and field increases with redshift, peaking at z ~ 2, with groups consistently showing higher fractions. We suggest AGN feedback partially drives rapid quenching in high-mass galaxies, while mergers may trigger AGN activity.