Statistics of active galactic nuclei in rich clusters revisited

Using the spectrophotometry of a large sample of galaxies in 19 Abell clust ers, we have selected 42 candidate active galactic nuclei (AGNs) using the criteria used by Dressier and coworkers in their analysis of the statistics of 22 AGNs in 14 rich cluster fields, which are based on the equivalent wi dth of [O II] 3727 Angstrom H beta, and [O III] 5007 Angstrom emission. We have then discriminated AGNs from H II region-like galaxies (hereafter H II galaxies) in the manner developed by Veilleux & Osterbrock using the addit ional information provided by H alpha and [N II] 6583 Angstrom or H alpha a nd [S II] 6716 + 6731 Angstrom emission, in order to test the reliability o f the selection criteria used by Dressier and coworkers. We find that befor e we discriminate AGNs from H II galaxies, our sample is very similar to th at of Dressier and coworkers and it leads to similar conclusions. However, we find that their method inevitably mixes H II galaxies with AGNs, even fo r the most luminous objects in our sample. We estimate a contamination of a t least 38% at a formal 90% confidence level. Since the study of Dressier a nd coworkers, other authors have attempted to quantify the relative fractio n of cluster-to-field AGNs and have reached similar conclusions, kbut they have used criteria similar to Dressier and coworkers to select AGNs (or hav e used the [O III] 5007 Angstrom/H beta flux ratio test that also mixes H I I galaxies with AGNs). Our sample of true AGNs remains too small to reach s tatistically meaningful conclusions, therefore a new study with a more time -consuming method that includes the other lines will be required to quantif y the true relative fraction of cluster-to-field AGNs.