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

¹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, C. Martí i Franquès 1, E-08028 Barcelona, Spain
² School of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
³ Departament de Física Quàntica i Astrofísica, Universitat de Barcelona, C. Martí i Franquès 1, E-08028 Barcelona, Spain

^⋆ Corresponding author: mathias.gort173@gmail.com

Received: 27 March 2025
Accepted: 3 April 2025

Abstract

Lenticular (S0) galaxies are ubiquitous in both high- and low-density environments where diverse evolutionary mechanisms operate. Consequently, studying their distribution and properties across both the dense, virialized cluster cores and their sparser surrounding secondary infall regions can provide key insights into the still-debated processes driving their evolution. In this work, we investigated the environmental impact of cluster regions on the evolution of present-day S0 galaxies, focusing on their distinct quiescent and star-forming (SF) subpopulations. We selected a sample of nearby cluster regions 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 was then generated from the stack of maximally relaxed clusters up to 3 virial radii to assess the locations of quiescent and SF S0s and their cluster infall histories. Additionally, we compared 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 their SF counterparts are more abundant in the outskirts. Despite this segregation, quiescent and SF S0s exhibit similar VDLOS profiles in the dynamically relaxed 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 ∼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 radial SSFR profiles that star formation in S0s begins to decline well beyond the virialized core, likely due to preprocessing in infalling groups. Overall, 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 (≲0.1 Gyr) after their first pericenter passage.