The distribution of lenticular galaxies in the phase space of present-day galaxy cluster regions

Abstract

We investigate the environmental impact of cluster regions on the evolution of nearby S0 galaxies, focusing on their distinct quiescent and star-forming (SF) subpopulations. We select a sample of clusters by crossmatching optical and X-ray data and extract a subset of 14 systems with maximally relaxed cores by applying strict virialization and substructure tests. A projected phase space (PPS) diagram is generated from the stack of relaxed clusters up to 3 virial radii to assess the locations of quiescent and SF S0s and their cluster infall histories. Additionally, we compare the radial line-of-sight velocity dispersion (VDLOS) and specific star-formation rate (SSFR) profiles for the different S0 subpopulations, using other Hubble types as benchmarks. Our study shows that quiescent S0s, the dominant class in the entire cluster region, concentrate preferentially at low radii in the PPS diagram, while SF S0s are more abundant in the outskirts. Despite this segregation, both subpopulations show similar VDLOS profiles in the cluster core -indicating an advanced stage of dynamical relaxation-, but that resemble those of late-type galaxies beyond the virial radius. This finding, combined with the distinct PPS distributions of both S0 subpopulations, which lead to mean infall times $\sim 1$ Gyr longer for quiescent S0s but that are shorter than those expected for ancient infallers, suggests that a substantial fraction of S0s present in the core region arrive via secondary infall. We also find evidence in the SSFR profiles that star formation in S0s begins to decline beyond the virialized core, likely due to preprocessing in infalling groups. Our results support a delayed-then-rapid quenching scenario for SF S0s in cluster regions, where their centrally concentrated star formation persists for an extended period before abruptly ending ($\lesssim 0.1$ Gyr) after their first pericenter passage.

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ORCID for J. L. Tous: orcid.org/0000-0002-1569-0468

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