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 \sigma _ { \mbox { { \small v } } } : the true fraction decreases from 0.12 for \mbox { $ \sigma _ { \mbox { { \small v } } } $ } \la 600 km s ^ { -1 } to 0.08 for \mbox { $ \sigma _ { \mbox { { \small v } } } $ } \ga 900 km s ^ { -1 } .

In only 12 out of 57 clusters , the average velocity of the ELG differs by more than 2 \sigma from that of the other galaxies , and in only 3 out of 18 clusters \sigma _ { \mbox { { \small v } } } of the ELG differs by more than 2 \sigma from that of the other galaxies .
Yet , combining the data for 75 clusters , we find that \sigma _ { \mbox { { \small 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 w.r.t .
the other galaxies ; instead , the larger \sigma _ { \mbox { { \small v } } } for 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 \sim 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 \sigma _ { \mbox { { \small v } } } and 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 .