2007ApJ...664L...9E

We present the first measurement of the AGN fraction in high-redshift clusters of galaxies (z∼0.6) with spectroscopy of one cluster and archival data for three additional clusters. We identify eight active galactic nuclei (AGNs) in these clusters from Chandra data that are sensitive to AGNs with hard X-ray (2-10 keV) luminosity L_X,H>10⁴³ ergs/s in host galaxies more luminous than a rest-frame M_R←20 mag. This stands in sharp contrast to the one AGN with L_X,H>10⁴³ ergs/s that we discovered in our earlier study of eight low-redshift clusters with z=0.06-0.31 (z{d1}∼0.2). Three of the four high-redshift cluster data sets are sensitive to L_X,H>10⁴² ergs/s, and we identify seven AGNs above this luminosity limit, compared to two AGNs in eight low-redshift clusters. Based on membership estimates for each cluster, we determine that the AGN fractions at z∼0.6 are f_A(L_X>10⁴²;M_R←20)=0.028^+0.019_–0.012 and f_A(L_X>10⁴³;M_R←20)=0.020^+0.012_–0.008. These values are approximately a factor of 20 greater than the AGN fractions in lower redshift (z{d1}∼0.2) clusters of galaxies. The cluster increase also represents substantially faster evolution than observed in the field over this redshift range (a factor of 1.3 and 3.3 for L_X>10⁴² and L_X>10⁴³, respectively). The cluster AGN fraction increases more rapidly with redshift than the field, and the increase in cluster AGNs indicates the presence of an AGN Butcher-Oemler effect.