10 - The duty cycle of radio galaxies and AGN feedback

Summary

Introduction

In recent years, remarkably tight correlations have been observed between the properties of supermassive black holes (SMBHs) residing in galaxy cores and those of the host galaxies themselves (Magorrian et al. 1998; Gebhhardt et al. 2000; Häring and Rix 2004). A growing body of evidence seems to support the idea that feedback from active galactic nuclei (AGN) provides a natural link between these. While every galaxy can potentially host a SMBH, only a relatively small fraction of these are observed in an active state. AGN activity manifests itself through powerful outflows observable right across the electromagnetic spectrum.

The central black holes are powered by accretion of surrounding cold gas. The resultant outflows, in turn, affect the cold gas supply by heating and/or transporting this gas away from dense inner regions with short cooling times. It is for this reason that feedback from radio sources is particularly interesting. Despite only contributing around one per cent of the AGN bolometric luminosity, radio-loud AGNs can profoundly affect their surroundings through such mechanical feedback. One piece of observational evidence supporting this view comes from studies of X-ray clusters. In the absence of feedback, large amounts of cold gas are expected in dense cluster cores (due to short cooling times), however no such gas has been found. This well-known ‘cooling flow problem’ points to the need for a central, powerful heating source.