Segregations in clusters of galaxies

Abstract

We analyse a sample of about 2000 galaxies in 40 regular clusters, to look for evidence of segregation with respect to galaxy luminosities and morphological types. We find evidence of luminosity segregation for galaxies brighter than $M_R < -21.5$, i.e. typically the four brightest members of each cluster. We also find evidence of morphological segregation: both the core-radius and the velocity dispersion increase along the Hubble sequence (ellipticals - S0 - early spirals - late spirals). Galaxies of different types have different velocity dispersion profiles, being steeper for later type galaxies. Simple modelling allows us to show that elliptical (and, to a lesser extent, S0) orbits are mostly tangential in the cluster core, and nearly isotropic outside, while spiral (in particular late-spiral) orbits are predominantly radial. A viable interpretation of our results is that (1) late spirals, at variance with other type galaxies, are a non-virialized cluster population, still on partially radial infalling orbits, (2) the elliptical phase-space distribution is evolving towards energy equipartition through the process of dynamical friction, (3) S0 and early-spirals have intermediate distributions between these two extremes.