The AGN Butcher-Oemler Effect

Paul Martini (The Ohio State University) , G. Sivakoff (OSU), J. Mulchaey (OCIW), D. Kelson (OCIW)

We have studied the evolution of the AGN population in clusters of galaxies with spectroscopic observations of X-ray sources in archival Chandra data. Our low-redshift sample of eight z=0.06-0.3 clusters of galaxies contains only one AGN more luminous than 10^43 erg/s in the hard X-ray band, while in contrast there are eight AGN above this luminosity range in a sample of four clusters at z=0.6. This is strong evidence for a pronounced increase in the fraction of AGN in higher-redshift clusters of galaxies, analogous to the increase in the number of blue galaxies in clusters at higher redshift known as the Butcher-Oemler effect. The AGN evolution is clusters is also more rapid than observed for field AGN, which indicates differential evolution in the cluster AGN population relative to the field, and not merely a different normalization in the number of AGN per galaxy.