Astron. Astrophys. 321, 84-104 (1997)

1. Introduction

Galaxies of different morphological types live in different environments (e.g. Hubble & Humason 1931). Dressler (1980a) was the first to clearly establish the dependence of the fractions of early- and late-type cluster galaxies on the local galaxy density. The dependence found by Dressler has been verified by many authors, most recently by e.g. Binggeli, Tarenghi & Sandage (1990); Sanromà & Salvador-Solé (1990); Iovino et al. (1993).

Early and late-type cluster galaxies not only differ in their spatial distribution, but also in their kinematics. Moss & Dickens (1977) claimed that the velocity dispersion, , of the population of late-type galaxies is significantly larger than that of the early-type galaxies, in 4 of the 5 clusters for which they could determine velocity dispersions for early- and late-type galaxies separately. Their study was a follow-up of earlier suggestions that the kinematics of early- and late-type galaxies in the Virgo cluster are different. Differences in average velocity, v (de Vaucouleurs 1961), as well as in (Tammann 1972) had been reported. Only the -difference was subsequently confirmed (Binggeli, Tammann, & Sandage 1987). The early claim of Moss & Dickens (1977) was confirmed by Sodré et al. (1989) and Biviano et al. (1992), from data on galaxies in 15 and 37 galaxy clusters respectively.

In clusters, the dependence of the mix of morphological types on local density (i.e. on distance from the cluster center), and the differences in kinematics that are related to this, can generally be understood as the result of the evolution of the galaxy population. Several processes may affect the morphology of a galaxy as it passes through the dense cluster core (e.g. ram pressure, merging, tidal stripping and tidal shaking). These processes are believed to be capable of transforming a star-forming spiral galaxy in a quiescent elliptical or S0. On the other hand, it is possible that regions of high density are, from the start, more conducive to the formation of slowly spinning (early-type ?) galaxies (see e.g. Sarazin 1986, and reference therein). It is likely that clusters form mainly through the collapse of density perturbations (e.g. Gunn & Gott 1972) although it is possible that shear also plays a rôle. If such density perturbations have density profiles that fall with radius, it is natural to expect a time sequence of infalling shells of galaxies. The spirals could then be on infalling orbits, as was convincingly shown to be the case in the Virgo cluster by Tully & Shaya (1984), whereas the ellipticals and S0's would constitute the virialized cluster population.

Some recent findings indicate that the latter scenario may well be too simplistic. On the one hand, Zabludoff & Franx (1993) have found that the early- and late-type galaxies have different average velocities in three out of six clusters studied, while the 's are not different. On the other hand, Andreon (1994) carefully re-examined galaxy morphologies in the Perseus cluster, and did not find a clear morphology-density relation. If groups of galaxies fall into a cluster anisotropically (as suggested e.g. by van Haarlem & van de Weygaert 1993), this may result in an average velocity of the infalling (spiral?) population that differs from that of the other galaxies in the (core of the) cluster. The resulting substructure could, at the same time, wash out the morphology-density relation.

Previous investigations of emission-line galaxies (ELG) in and outside clusters (Gisler 1978; Dressler, Thompson & Shectman 1985; Salzer et al. 1989; Hill & Oegerle 1993; Salzer et al. 1995) have been mainly limited to the comparison of the relative frequency of ELG in clusters and in the field. These studies have shown that emission lines occur more frequently in the spectra of field galaxies than in cluster galaxies (for elliptical galaxies this was already pointed out by Osterbrock, 1960). It was concluded that this difference cannot totally be the result of the morphology-density relation, in combination with the different mix of early- and late-type galaxies. However, recently the kinematics of the ELG has become a subject of study (e.g. Mohr et al. 1996; Carlberg et al. 1996).

In this paper, we re-examine the evidence for differences between early- and late-type galaxies in clusters, by using the extensive data-base provided by the ENACS (the ESO Nearby Abell Cluster Survey). We analyze the frequency of occurrence of ELG in clusters, as well as their distribution with respect to velocity and position and their kinematics. In Sect.  2 we summarize those properties of the ENACS data-base that are relevant for the present discussion. In Sect.  3 we discuss the fraction of ELG in clusters and in the field. In Sect.  4 and Sect.  5 we study the global kinematics and spatial distribution of the ELG in relation to the non-ELG. In Sect.  6 we discuss correlations between positions and velocities of the ELG and non-ELG. In Sect.  7 we investigate the equilibrium and the orbits of the cluster galaxies, and, finally, in Sect.  8 we discuss the implications of our results for ideas about structure and formation of clusters.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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