THE ESO NEARBY ABELL-CLUSTER SURVEY .3. DISTRIBUTION AND KINEMATICS OF EMISSION-LINE GALAXIES

We have used the ESO Nearby Abell Cluster Survey (ENACS) data, to inve stigate 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 est imate that the detected ELG represent only about one-third of the tota l spiral population. The apparent fraction of ELG increases towards fa inter magnitude, as redshifts are more easily obtained from emission l ines 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 tote fraction is 0.21. The true ELG fract ions in field and clusters are consistent if the differences in morpho logical mix are taken into account. Thus, it is not necessary to assum e that ELG in and outside clusters have different emission-line proper ties. The average ELG fraction in clusters depends on global velocity dispersion sigma(v): the true fraction decreases from 0.12 for <sigma( v) less than or similar to) 600 km s(-1) to 0.08 for sigma(v) greater than or similar to 900 km s(-1). In only 12 out of 57 clusters, the av erage velocity of the ELG differs by more than 2 sigma from that of th e other galaxies, and in only 3 out of 18 clusters sigma(v) of the ELG differs by mon than 2 sigma from that of the other galaxies. Yet, com bining the data for 75 clusters, we find that sigma(v) of 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 sigma(v) for the ELG must be largely intrinsic. The spatial distribution of the ELG is significa ntly less peaked towards the centre than that of the other galaxies. T his causes the average projected density around ELG to be similar to 3 0% lower than it is around the other galaxies. In combination with the inevitable magnitude bias against galaxies without detectable emissio n lines, this can lend to serious systematic effects in the study of d istant clusters. From an analysis of the distributions of projected pa ir distances and velocity differences we conclude that at most 25% of the ELG are in compact substructures, while the majority of the ELG ar e distributed more or less smoothly. The virial estimates of the clust er masses based on the ELG only are, on average, about 50% higher than those derived from the other galaxies. This indicates that the ELG ar e 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 i s found to be consistent with the ELG being on more radial orbits than the other galaxies. For the ELG, a ratio between tangential and radia l velocity dispersion of 0.3 to 0.8 seems most likely, while for the o ther galaxies the data are consistent with isotropic orbits. The lower amount of central concentration, the larger value of sigma(v) and the possible orbital anisotropy of the ELG, as well as their content of l ine-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) appro ach towards the central, high-density region.