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

TL;DR

This paper investigates how dense galaxy groups influence star formation and morphology over cosmic time by analyzing the 25 richest COSMOS-Web groups ($0.18<z<3.65$) identified with AMICO and cross-matched with JWST/COSMOS-Web data. The authors combine multi-wavelength observations, including HLAGN from VLA-COSMOS and spectroscopic redshifts, to quantify central overdensities, red sequences, AGN activity, and the stellar mass–SFR landscape, revealing that quenching typically precedes morphological transformation within groups. A two-channel quenching picture emerges: rapid, mass-driven quenching in high-mass galaxies ($M_\\star>10^{10.5} M_\odot$) leading to spheroidal ETGs, and slower, environment-driven quenching for intermediate-to-low-mass systems that retain disk morphologies as they quench; red sequences are present in several groups, with LTGs contributing ~13% of RS members, signaling disks quenched while maintaining disks. The work highlights the transitional role of groups in galaxy evolution, showing concurrent passive-density and active-density trends, associating HLAGN activity with cosmic noon, and providing a framework to test quenching pathways with future Euclid-wide data and hydrodynamical simulations.

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.

Figures (14)

• Figure 1: Intrinsic richness $\lambda_\star$ versus redshift (Hexbin map) for the COSMOS-Web AMICO detections. Each bin is colored by the median $\texttt{SN\_NO\_CLUSTER}$, the dashed red line marks the richness threshold $\lambda_\star=10.81$, and the purple outlines highlight the 25 groups retained for detailed analysis.
• Figure 2: Member-selection diagnostics for group ID 4. Left: redshift distribution of AMICO candidates. The red solid line and hatched region (bounded by green lines) represent $z_{\mathrm{DET}}$ and the adopted selection window $\Delta z = \pm 0.01(1 + z_{\mathrm{DET}})$, respectively. Middle: projected distribution of LTGs (blue) and ETGs (red) in concentric annuli, with galaxies having $\text{assoc\_prob} > 0.5$ shown as filled circles and those with $\text{assoc\_prob} \leq 0.5$ as unfilled circles; matched HLAGN are marked as stars colored by $|z_{\mathrm{AGN}}-z_{\mathrm{DET}}|$. Right: radial surface-density profile showing ETG, LTG, and combined overdensities after subtracting the local background (PER). Panels in the bottom row repeat the analysis for high field-probability galaxies (e.g. FIELD_PROB > 0.7). Negative values indicate underdensities, where the accumulated density minus the outskirts baseline becomes negative.
• Figure 3: Average radius $R_{\mathrm{cut}}$ as a function of redshift. Blue points show binned means with Poisson errors, and the dashed curve denotes the quadratic fit $R_{\mathrm{cut}}=-22.43 z^2 - 69.27 z + 791.11\,\mathrm{kpc}$.
• Figure 4: Average overdensity versus redshift for ETGs (red circles) and LTGs (blue squares), and their sum (black), measured within cumulative apertures of $200\,\mathrm{kpc}$. Error bars denote the standard error of the mean, while the dashed curve and shaded regions show the Markov-Chain Monte Carlo (MCMC) fit and its 68% credible interval for the combined population. Average overdensity of field galaxies is consistent with a constant background level.
• Figure 5: Fraction of HLAGNs as a function of redshift for central ($r < 0.2\,\mathrm{Mpc}$) and outskirt ($0.2 < r < 1.0\,\mathrm{Mpc}$) regions. Points show the combined group sample with Poisson uncertainties.