The ESO Nearby Abell Cluster Survey. III. Distribution and Kinematics of Emission-Line Galaxies

Abstract

We have studied the frequency of occurrence, the kinematics and the spatial distribution of Emission-Line Galaxies (ELG) in clusters in the ESO Nearby Abell Cluster Survey (ENACS). More than 90% of the ELG are spirals, but the ELG represent only 30% of all spirals. The true ELG fraction in clusters is 0.10; this is consistent with that in the field (0.21) as a result of differences in morphological mix. The average ELG fraction decreases with increasing velocity dispersion of the cluster. Only 1/5 of the clusters shows a significant difference in the average velocities of the ELG and the other (non-ELG) galaxies; in a similar fraction of the clusters significant differences in velocity dispersion are seen. Yet, the velocity dispersion of the ELG is, on average, 20% larger than that of the non-ELG. This larger velocity dispersion is largely intrinsic, i.e. it is not likely to be the result of velocity offsets of the ELG. The surface density of the ELG is significantly less peaked towards the centre than that of the non-ELG. The ELG are distributed fairly smoothly within the cluster with at most 25% in compact substructures. The distribution and kinematics of the ELG yield virial mass estimates that, on average, are 50% higher than those based on the non-ELG. This probably means that the ELG are on more radial orbits than the non-ELG. All observed properties of the ELG are consistent with most of them being on their first approach of the high-density cluster core.