Radio AGN in galaxy clusters: heating hot atmospheres and driving supermassive black hole growth over cosmic time

Abstract

We estimate the average radio-AGN (mechanical) power deposited into the hot atmospheres of galaxy clusters over more than three quarters of the age of the Universe. Our sample was drawn from eight major X-ray cluster surveys, and includes 685 clusters in the redshift range 0.1 < z < 0.6 that overlap the area covered by the NVSS. The radio AGN mechanical power was estimated from the radio luminosity of central NVSS sources, using the relation of Cavagnolo et al. 2010 that is based on mechanical powers determined from the enthalpies of X-ray cavities. We find only a weak correlation between radio luminosity and cluster X-ray luminosity, although the most powerful radio sources resides in luminous clusters. The average AGN mechanical power of 3x10^{44} erg/s exceeds the X-ray luminosity of 44% of the clusters, indicating that the accumulation of radio-AGN energy is significant in these clusters. Integrating the AGN mechanical power to redshift z=2, using simple models for its evolution and disregarding the hierarchical growth of clusters, we find that the AGN energy accumulated per particle in low luminosity X-ray clusters exceeds 1 keV per particle. This result represents a conservative lower limit to the accumulated thermal energy. The estimate is comparable to the level of energy needed to "preheat" clusters, indicating that continual outbursts from radio-AGN are a significant source of gas energy in hot atmospheres. Assuming an average mass conversion efficiency of $\eta=0.1$, our result implies that the supermassive black holes that released this energy did so by accreting an average of ~10^9 M_{sun} over time, which is comparable to the level of growth expected during the quasar era.