1997A&A...321...84B

We have used the ESO Nearby Abell Cluster Survey (ENACS) data, to investigate the frequency of occurrence of Emission-Line Galaxies (ELG) in clusters, as well as their kinematics and spatial distribution. Well over 90% of the ELG in the ENACS appear to be spirals; however, we estimate that the detected ELG represent only about one-third of the total spiral population. The apparent fraction of ELG increases towards fainter magnitude, as redshifts are more easily obtained from emission lines than from absorption lines. From the ELG that have an absorption-line redshift as well, we derive a true ELG fraction in clusters of 0.10, while the apparent fraction is 0.16. The apparent ELG fraction in the field is 0.42, while the true fraction is 0.21. The true ELG fractions in field and clusters are consistent if the differences in morphological mix are taken into account. Thus, it is not necessary to assume that ELG in and outside clusters have different emission-line properties. The average ELG fraction in clusters depends on global velocity dispersion σ_v: the true fraction decreases from 0.12 for σ_v≲600km/s to 0.08 for σ_v>900km/s. In only 12 out of 57 clusters, the average velocity of the ELG differs by more than 2σ from that of the other galaxies, and in only 3 out of 18 clusters σ_vof the ELG differs by more than 2σ from that of the other galaxies. Yet, combining the data for 75 clusters, we find that σ_vof the ELG is, on average, 20% larger than that of the other galaxies. It is unlikely that this is primarily due to velocity offsets of the ELG with regard to the other galaxies; instead, the larger σ_vfor the ELG must be largely intrinsic. The spatial distribution of the ELG is significantly less peaked towards the centre than that of the other galaxies. This causes the average projected density around ELG to be ∼30% lower than it is around the other galaxies. In combination with the inevitable magnitude bias against galaxies without detectable emission lines, this can lead to serious systematic effects in the study of distant clusters. From an analysis of the distributions of projected pair distances and velocity differences we conclude that at most 25% of the ELG are in compact substructures, while the majority of the ELG are distributed more or less smoothly. The virial estimates of the cluster masses based on the ELG only are, on average, about 50% higher than those derived from the other galaxies. This indicates that the ELG are either on orbits that are significantly different from those of the other galaxies, or that the ELG are not in virial equilibrium with the other galaxies, or both. The velocity dispersion profile of the ELG is found to be consistent with the ELG being on more radial orbits than the other galaxies. For the ELG, a ratio between tangential and radial velocity dispersion of 0.3 to 0.8 seems most likely, while for the other galaxies the data are consistent with isotropic orbits. The lower amount of central concentration, the larger value of σ_vand the possible orbital anisotropy of the ELG, as well as their content of line-emitting gas would be consistent with a picture in which possibly all spirals (but certainly the late-type ones) have not yet traversed the virialized cluster core, and may even be on a first (infall) approach towards the central, high-density region.