X-RAY POINT SOURCES AND RADIO GALAXIES IN CLUSTERS OF GALAXIES

Abstract

Using Chandra imaging spectroscopy and Very Large Array L-band maps, we have identified radio galaxies at P (1.4 GHz)≥ 3 × 1023 W Hz–1 and X-ray point sources (XPSs) at L (0.3-8 keV)≥ 1042 erg s–1 in 11 moderate redshift (0.2 99.999% level. We suggest that these active galactic nuclei (AGNs) are triggered somehow by the intracluster medium (ICM), perhaps similar to the Bondi accretion model of Allen et al. All but one of the XPSs are hosted by luminous ellipticals which otherwise show no other evidence for AGN activity. These objects are unlikely to be highly obscured AGN since there is no evidence for large amounts of X-ray or optical absorption. One XPS, in addition to possessing a pure absorption-line optical spectrum, has a large excess of light blueward of the Ca II HK a second XPS host galaxy probably has excess blue light. The most viable model for these sources are low-luminosity BL Lac Objects, similar to the high-energy-peaked BL Lacs (HBLs) discovered in abundance in serendipitous X-ray surveys. The expected numbers of lower luminosity FR 1 radio galaxies and HBLs in our sample converge to suggest that very deep radio and X-ray images of rich clusters will detect AGN (either X-ray or radio emitting or both) in a large fraction of bright elliptical galaxies in the inner 500 kpc. Because both the radio galaxies and the XPSs possess relativistic jets, they (and, by extension, the entire radio luminosity function) can inject heat into the ICM. Using the most recent scalings of P jet L 0.5 r from Birzan et al., radio sources weaker than our luminosity limit probably contribute the majority of the heat to the ICM. Also, because these heat sources move around the cluster, AGN heating is distributed rather evenly. If a majority of ICM heating is due to large numbers of low-power radio sources, triggered into activity by the increasing ICM density as they move inward, this may be the feedback mechanism necessary to stabilize cooling in cluster cores.