Star formation quenching precedes morphological transformation in COSMOS-Web’s richest galaxy groups

We analyzed the 25 richest galaxy groups in COSMOS-Web across the redshift range z = 0.18–3.65, identified using the Adaptive Matched Identifier of Clustered Objects (AMICO) algorithm. The groups have about 20–30 galaxies with a high (> 75%) membership probability. Our study reveals both passive-density and active-density relations, with late-type galaxies (LTGs) preferring both higher central overdensities than early-type galaxies (ETGs) across all groups, and secondly many massive LTGs have colors typical for quiescent galaxies. We identify red sequences (RS) in five out of 25 galaxy groups, prominently established at z < 1, with early emergence in the RS locus up to z ∼ 2.2. Although LTGs outnumber ETGs, ETGs are quenched much more efficiently than LTGs up to z ∼ 3.5. This finding suggests that group environments represent a transitional phase where star formation quenching precedes morphological transformation, contrasting with the classical morphology–density relation observed in rich clusters. In the central regions of the identified groups, within a radius of ∼33″ (100 kpc) from the group centers, we identified 86 galaxies. Among them, 23 (∼27%) were classified as ETGs and 63 (∼73%) as LTGs. High-mass galaxies ( M ★ > 10 10.5 M ⊙ ) undergo rapid, transformative quenching over ∼1 Gyr, becoming predominantly spheroidal ETGs, indicating that morphological transformation accelerates dramatically in the most massive systems during the epoch of peak cosmic star formation. Intermediate-mass galaxies (10 9 < M ★ /M ⊙ < 10 10.5 ) show mild quenching, while low-mass galaxies ( M ★ < 10 9 M ⊙ ) remain largely star-forming, with environmental processes gradually suppressing star formation without destroying disk structures, suggesting that environmental quenching in groups operates on longer timescales than mass quenching. Overall, mass-dependent quenching dominates at the high-mass end, while environment-driven quenching shapes lower-mass systems, highlighting the dual nature of galaxy evolution across cosmic time. The fraction of high−luminosity active galactic nuclei (HLAGN) for both group and field galaxies increases with redshift, peaking at z ∼ 2, with groups consistently showing a higher AGN fraction than field. We suggest that AGN feedback plays a partial role in the rapid cessation of star formation in high-mass galaxies, while mergers may contribute to triggering AGN activity.