X-ray Point Sources and Radio Galaxies in Clusters of Galaxies

TL;DR

This study identifies and characterizes radio galaxies and X-ray point sources in galaxy clusters, suggesting they play a significant role in heating the intracluster medium and stabilizing cooling flows.

Contribution

It provides the first detailed analysis of low-luminosity AGN in clusters at moderate redshift, linking radio and X-ray sources to potential feedback mechanisms.

Findings

75% of sources are within 500 kpc of cluster centers

Most X-ray point sources are hosted by luminous ellipticals with no other AGN evidence

Low luminosity radio sources likely contribute major heating to the intracluster medium

Abstract

Using Chandra imaging spectroscopy and VLA L-band maps, we have identified radio galaxies at P(1.4 GHz) >= 3x10^{23} W Hz^{-1} and X-ray point sources (XPSs) at L(0.3-8 keV) >= 10^{42} ergs s^{-1} in 11 moderate redshift (0.2<z<0.4) clusters of galaxies. Each cluster is uniquely chosen to have a total mass similar to predicted progenitors of the present-day Coma Cluster. Within a projected radius of 1 Mpc we detect 20 radio galaxies and 8 XPSs confirmed to be cluster members above these limits. 75% of these are detected within 500 kpc of the cluster center. This result is inconsistent with a random selection from bright, red sequence ellipticals at the > 99.999% level. 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. The most viable model for these sources are low luminosity BL Lac Objects. The expected numbers of lower luminosity FR 1 radio galaxies and BL Lacs in our sample converge to suggest that very deep radio and X-ray images of rich clusters will detect AGN 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 can inject heat into the ICM. Using the most recent scalings of P_jet ~ L_radio^{0.5} from Birzan et al. (2008), radio sources weaker than our luminosity limit probably contribute the majority of the heat to the ICM. 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.