COSMOS-Web galaxy groups: Evolution of red sequence and quiescent galaxy fraction
February 2026 • 2026A&A...707A..87T
Abstract • Aims. We investigate the redshift evolution and group richness dependence of the quiescent galaxy fraction and red sequence (RS) parameters in COSMOS galaxy groups, spanning a wide redshift range, from z = 0 to z = 3.7. Methods. We analyzed the deep and well-characterized sample of groups recently detected with the AMICO algorithm in the COSMOS(-Web) field. Our study of the quiescent galaxy population is based on a machine-learning classification tool based on rest-frame magnitudes. The algorithm learns from several traditional methods to estimate the probability of a galaxy being quiescent, achieving high precision and recall. Starting from this classification, we computed quiescent galaxy fractions within groups via two methods: one based on the membership probabilities provided by AMICO, which rely on an analytical model, and another using a model-independent technique. We then detected the RS by estimating the ridgeline position using probability-weighted photometric data, followed by σ clipping to remove outliers. This analysis was performed using both rest-frame magnitudes and observer-frame magnitudes with rest-frame matching. We compared the results from both approaches and investigated the redshift and richness dependence of the RS parameters. Results. We found that the quiescent galaxy population in groups builds up steadily from z = 1.5 − 2 across all richnesses, with faster and earlier growth in the richest groups. The first galaxies settle onto the RS ridgeline by z ∼ 2, consistent with current evolutionary scenarios. Notably, we reported a rare protocluster core hosting quiescent galaxies at z = 3.4, potentially one of the most distant early RSs observed. Extending our study to X-ray properties, we found that X-ray faint groups have, on average, lower quiescent fractions than X-ray bright ones, likely reflecting their typical location in filaments where pre-processing is lower. Leveraging the broad wavelength coverage of COSMOS2025, we traced RS evolution using observed and rest-frame colors over ∼12 Gyr, finding no significant trends in either the slope or the scatter of the ridgeline.
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