Accretion and feedback from supermassive black holes in galaxy clusters

Abstract

A significant fraction of galaxy clusters, namely the cool-core clusters (CCCs), exhibit a dip in their central temperature profiles, with radiative cooling times much shorter than the Hubble time. Unchecked, radiative cooling of this magnitude is expected to cause the accumulation of cold gas at the cluster center that leads to star formation rates 100-1000 times higher than those inferred by observations. This discrepancy suggests the existence of active heating mechanisms that counteract the overcooling in cluster centers. The dominant mechanism has now been widely recognized as the kinetic feedback from the radio-loud active galactic nuclei (AGNs). Recent observations however find substantial amounts of cold gas in a number of CCCs, as well as evidence that some clusters host quasars in their brightest cluster galaxies, raising questions about the significance of radiative feedback in such systems. In this work, we use 3D high-resolution radiation-hydrodynamic simulations with the code Enzo to investigate how AGN feedback operates in CCCs and how it couples to the intracluster medium. The new element of this work is an in-depth examination of the role of the radiative feedback and its impact on accretion, kinetic feedback, presence of cold gas, and other physical processes that play out in CCCs. Our main findings strongly suggest that (a) central AGNs in CCCs transition between the radiatively efficient and radiatively inefficient states on timescales of a few Gyr, powered by accretion of cold gas, (b) kinetic feedback must be present at both low and high accretion rates in order to prevent the cooling catastrophe, and (c) that there is a positive correlation between the AGN feedback power and properties of the cold gas filaments in the cluster core, indicating that they can be used to probe the history of AGN activity.