First Measurement of a Rapid Increase in the Agn Fraction in High-redshift Clusters of Galaxies

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 8 AGN in all four of these clusters from the Chandra data, which are sensitive to AGN with hard X-ray (2-10keV) luminosity LX ,H > 1043 erg s−1 in host galaxies more luminous than a rest frame MR < −20 mag. This stands in sharp contrast to the one AGN with LX ,H > 1043 erg s−1 we discovered in our earlier study of eight low-redshift clusters with z = 0.06 → 0.31(z̄ ∼ 0.2). Three of the four high-redshift cluster datasets are sensitive to nearly LX ,H > 1042 erg s−1 and we identify seven AGN above this luminosity limit, compared to two in eight, lowredshift clusters. Based on membership estimates for each cluster, we determine that the AGN fraction at z ∼ 0.6 is fA(LX > 10;MR < −20) = 0.028+0.019 −0.012 and fA(LX > 10 ;MR < −20) = 0.020+0.012 −0.008. These values are approximately a factor of 20 greater than the AGN fractions in lower-redshift (z̄ ∼ 0.2) clusters of galaxies and represent a substantial increase over the factors of 1.5 and 3.3 increase, respectively, in the measured space density evolution of the hard X-ray luminosity function over this redshift range. Potential systematic errors would only increase the significance of our result. The cluster AGN fraction increases more rapidly with redshift than the field and the increase in cluster AGN indicates the presence of an AGN Butcher-Oemler Effect. Subject headings: galaxies: active – galaxies:evolution – clusters: individual (MS2053-04, CL0542-4100, CL0848.6+4453, CL0016+1609)