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Astron. Astrophys. 317, 670-675 (1997)

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1. Introduction

One of the main results of recent galaxy cluster research is the unambiguous finding that these high-density structures are still continuing to be built or at least significantly reshaped. Undoubtedly, the discovery of substructures has played an important role, changing our understanding about the degree of dynamical evolution in galaxy clusters.
Several statistical treatments to measure the frequency of substructures have been presented during the last decade. Nevertheless, the question about the significance of substructure detections on small scales or in the core regions of clusters has been touched only in a few works (Fitchett & Webster 1987; Mellier et al. 1988; Escalera et al. 1992; Salvador-Solé et al. 1993).
It is interesting that the frequency and degree of clumpiness in the centers of clusters could be helpful in establishing the density profile of dark matter, and even allow an estimation of [FORMULA] (e.g. Richstone et al. 1992). If namely dark matter was strongly concentrated towards the cluster center one would expect tidal forces acting towards the disruption of subclumps (see González-Casado et al. 1994 for a discussion).
Examining the very centers of galaxy clusters is rewarding for another reason. The importance of accretion or "cannibalism" (Ostriker & Hausman 1977) during the formation of a cluster is not well established, because right in the cores of galaxy clusters the galaxy velocity dispersion is expected to be too high to allow an efficient dynamical friction (Merrifield & Kent 1989, Gebhardt & Beers 1991, Blakeslee & Tonry 1992). An attractive solution to this problem has been proposed by Merritt (1985).
As also numerical simulations suggest (West & Richstone 1988, Serna et al. 1994), dynamical friction could be an important mechanism during the evolution of galaxy clusters. If this is the case, it should be possible to detect signs of equipartition of kinetic energies for the most massive galaxies in dense regions, leading to a correlation of velocity dispersion with galaxy luminosity. Early observational studies of mass segregation were mostly limited to the positions of galaxies in projection, with the exception of e.g. Chincarini & Rood (1977), and no general agreement was foud about the relevance of luminosity segregation in velocity space. Recently, Biviano et al. (1992) once again stated the evidence for mass segregation in the velocity distribution of a merged cluster sample taken from the literature. Presumably, the phenomenon is easily overlooked in individual clusters studies because of the limited number of galaxies involved.
On the other side, a difference in the kinematic properties between early and late-type galaxies has also been detected (Binggeli et al. 1987, Sodré et al. 1989). This means that one should look separately at early and late-type galaxies, because galaxies which didn't take part in most of the cluster evolution are not expected to show signs of mass segregation.
This paper addresses the problem of mass segregation and central substructure based on a homogeneous sample of kinematical data (Stein 1996, hereafter Paper I). It is organized as follows: Sect. 2 gives a description of the data used, i.e. the source of redshifts, magnitudes and types for the galaxies, as well as cluster definition and selection. The analysis of substructures on individual clusters is done in Sect. 3, while luminosity segregation is measured on the merged cluster sample (Sect. 4). The results are then discussed in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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