2005ApJ...625L...9N

The radio source Hercules A resides at the center of a cooling flow cluster of galaxies at redshift z=0.154. A Chandra X-ray image reveals a shock front in the intracluster medium (ICM) surrounding the radio source, about 160 kpc from the active galactic nucleus (AGN) that hosts it. The shock has a Mach number of 1.65, making it the strongest of the cluster-scale shocks driven by an AGN outburst found so far. The age of the outburst is ≃5.9x10⁷ yr, its energy ∼3x10⁶¹ ergs, and its mean power ∼1.6x10⁴⁶ ergs/s. As for the other large AGN outbursts in cooling flow clusters, this outburst overwhelms radiative losses from the ICM of the Hercules A Cluster by a factor of ∼100. It adds to the case that AGN outbursts are a significant source of preheating for the ICM. Unless the mechanical efficiency of the AGN in Hercules A exceeds 10%, the central black hole must have grown by more than 1.7x10⁸ M_☉to power this one outburst.