The ESO Nearby Abell Cluster Survey ^*,**

XI. Segregation of cluster galaxies and subclustering

¹ INAF, Osservatorio Astronomico di Trieste, Italy
² Sterrewacht Leiden, The Netherlands
³ Laboratoire d'Astrophysique de Marseille, France

Corresponding author: A. Biviano, biviano@ts.astro.it

Received: 22 October 2001
Accepted: 30 January 2002

Abstract

We study luminosity and morphology segregation of cluster galaxies in an ensemble cluster built from 59 rich, nearby galaxy clusters observed in the ESO Nearby Cluster Survey (ENACS). The ensemble cluster contains 3056 member galaxies with positions, velocities and magnitudes; 96% of these also have galaxy types. From positions and velocities we identify galaxies within substructures, viz. as members of groups that are significantly colder than their parent cluster, or whose average velocity differs significantly from the mean. We compare distributions of projected clustercentric distance R and relative line-of-sight velocity v, of galaxy subsamples drawn from the ensemble cluster, to study various kinds of segregation, the significance of which is obtained from a 2-dimensional Kolmogorov-Smirnov test. We find that luminosity segregation is evident only for the ellipticals that are outside (i.e. not in) substructures and which are brighter than . This is mainly due to the brightest cluster members at rest at the centre of the cluster potential. We confirm the well-known segregation of early- and late-type galaxies. For the galaxies with of all types (E, S0, S and emission-line galaxies, or ELG, for short), we find that those within substructures have -distributions that differ from those of the galaxies that are not in substructures. The early and late spirals (Sa–Sb and Sbc–Ir respectively) that are not in substructures also appear to have different -distributions. For these reasons we have studied the segregation properties of 10 galaxy subsamples: viz. E, S0, S_e, S_l and ELG, both within and outside substructures. Among the 5 samples of galaxies that are not in substructures, at least 3 ensembles can and must be distinguished; these are: [E+S0], S_e, and [ S_l+ELG] . The [E+S0] ensemble is most centrally concentrated and has a fairly low velocity dispersion that hardly varies with radius. The [ S_l+ELG] ensemble is least concentrated and has the highest velocity dispersion, which increases significantly towards the centre. The class of the S_e galaxies is intermediate to the two ensembles. Its velocity dispersion is very similar to that of the [E+S0] galaxies in the outer regions but increases towards the centre. The galaxies within substructures do not all have identical -distributions; we need to distinguish at least two ensembles, because the S0 and [ S_l+ELG] galaxies have different distributions in R as well as in v. The [ S_l+ELG] galaxies are less centrally concentrated and, in the inner region, their velocity dispersion is higher than that of the S0 galaxies. Our data allow the other 3 galaxy classes to be combined with these two classes in 4 ways. We discuss briefly how our data provide observational constraints for several processes inside clusters, like the destruction of substructures, the destruction of late spirals and the transformation of early spirals into S0s.