The Kinematics of Emission-line Galaxies in Clusters from Enacs

By using the ENACS data, we have studied the properties of emission-line galaxies (ELG) in clusters. The fraction of ELG in clusters is 16 %; this fraction is slightly dependent on the cluster velocity dispersion, being larger for \colder" systems. ELG tend to occur more frequently than other cluster galaxies in substructures. However, only a minor cluster fraction has an ELG population with a mean velocity signiicantly diierent from that of the overall cluster population. The velocity distribution of cluster ELG is broader than that of the other cluster galaxies, which translates in a larger observed velocity dispersion. The velocity distribution for ELG in the inner cluster regions suggests that ELG are on mostly radial orbits. 1. INTRODUCTION Galaxies of diierent morphological types live in diierent environments (e.g. Hubble & Humason 1931): Dressler (1980a) clearly established the dependence between the fraction of early-and late-type cluster galaxies, and their local density. Early and late-type galaxies not only diier in their spatial distribution, but also in their kinematics, as rst shown by Moss & Dickens (1977), and later connrmed by Sodr e et al. (1989) and Biviano et al. (1992), for samples of 15 and 37 galaxy clusters , respectively. The segregation of morphological types with respect to their local density kinematics, indicates that either the formation process or the subsequent evolution of a galaxy or both are aaected by the galaxy environment. As a cluster probably forms through the collapse of a density perturbation with a radially decreasing density, a time sequence of infalling shells of galaxies is expected: the spirals would then be on infalling orbits while the ellipticals and S0's would constitute the virialized cluster population (Tully & Shaya 1984). The