Statistics of emission-line galaxies in rich clusters.

The emission-line characteristics of a sample of 1095 galaxies in rich clusters and 173 field galaxies for which Dressler and Shectman (1985) have obtained spectra are tabulated and analyzed. It is found that the emission-line frequency is much higher in field galaxies than in cluster galaxies. Relatively strong emission is found in the central regions of 31 percent of the field galaxies, but in only 7 percent of the cluster galaxies. The difference in the distribution of morphological types can only partially explain this effect; much of the difference must be attributed to environmental influences. Active galactic nuclei (AGNs) occur at a frequency of about 5 percent in the field sample but only about 1 percent in the cluster sample, the same ratio as found for the emission-line galaxies. The average frequency is an order of magnitude lower than that found for the distant 3C 295 cluster by Dressler and Gunn (1983), but some nearby clusters may also have an unusually high AGN fraction.