The phase-space distribution of present-day lenticular galaxies in relaxed galaxy clusters

Abstract

Aims. In this work we investigate the distribution of lenticular galaxies (S0s) in the projected phase space (PPS) of local, relaxed galaxy clusters. The aim is to shed further light on the still debated formation and evolutionary processes that shape S0s, by probing the properties of different subpopulations of actively star-forming (SF) and passive systems in which the heterogeneous class of lenticular galaxies can be divided. Methods. Optical and X-ray cluster identifications, along with tests to estimate the degree of virialization and substructure of galaxy associations, are used to select a sample of cluster regions with maximally relaxed cores. A PPS diagram is then constructed for this ensemble of clusters. Subsequently, a series of statistical tests are applied to the S0 member galaxies to compare the phase-space distributions of various S0 subpopulations and their times since cluster infall. Finally, line-of-sight velocity dispersion and specific star-formation rate (SSFR) radial profiles are constructed for both active and passive S0s. Results. Local, relaxed galaxy clusters contain ∼80% of passive S0s and ∼20% of star-forming S0s. The PPS distributions of these two classes of S0s are significantly different, with the passive systems concentrated closer to the cluster centers, and the active S0s being more abundant in the outer cluster regions.We find evidence that active S0s have been accreted more recently than passive S0s, with a gap in mean cluster infall time of about 1 Gyr. Despite these differences, the LOS velocity dispersions of these two subpopulations of S0s exhibit surprisingly similar projected radial profiles, consistent with radial infall. However, the SSFR radial profiles differ markedly: passive S0s show a flat profile up to three virial radii, while active lenticulars experience a sharp decline in SSFR between ∼2.5 and 1.5Rvir, followed by a more gradual decrease extending to the cluster center.