Statistics of Active Galactic Nuclei in Rich Clusters Revisited

Details Statistics of Active Galactic Nuclei in Rich Clusters Revisited Statistics of Active Galactic Nuclei in Rich Clusters Revisited

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...502..134w&link_type=abstract

Astrophysical Journal v.502, p.134

Astronomy and Astrophysics

Astronomy

6

Galaxies: Active, Galaxies: Clusters: General

Scientific paper

Using the spectrophotometry of a large sample of galaxies in 19 Abell clusters, we have selected 42 candidate active galactic nuclei (AGNs) using the criteria used by Dressler and coworkers in their analysis of the statistics of 22 AGNs in 14 rich cluster fields, which are based on the equivalent width of [O II] 3727 Angstroms, H beta , and [O III] 5007 Angstroms 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 additional information provided by H alpha and [N II] 6583 Angstroms or H alpha and [S II] 6716 + 6731 Angstroms emission, in order to test the reliability of the selection criteria used by Dressler and coworkers. We find that before we discriminate AGNs from H II galaxies, our sample is very similar to that of Dressler and coworkers and it leads to similar conclusions. However, we find that their method inevitably mixes H II galaxies with AGNs, even for the most luminous objects in our sample. We estimate a contamination of at least 38% at a formal 90% confidence level. Since the study of Dressler and coworkers, other authors have attempted to quantify the relative fraction of cluster-to-field AGNs and have reached similar conclusions, but they have used criteria similar to Dressler and coworkers to select AGNs (or have used the [O III] 5007 Angstroms/H beta flux ratio test that also mixes H II galaxies with AGNs). Our sample of true AGNs remains too small to reach statistically meaningful conclusions, therefore a new study with a more time-consuming method that includes the other lines will be required to quantify the true relative fraction of cluster-to-field AGNs.

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