Simona Giacintucci
Naval Research Laboratory
Extreme AGN outbursts in galaxy clusters
Clusters of galaxies are the largest, gravitationally-bound structures in the Universe. The space between the galaxies is filled by hot ionized gas -- the intra-cluster medium (ICM) -- that emits X-ray radiation. In the cores of many clusters, the ICM is dense enough to cool very rapidly, on a timescale shorter than the cluster’s lifetime. This cooling process should deposit large amounts of cold gas in the cluster centers. However, there is little evidence for such vast quantities of cool gas, suggesting a compensatory source of heat. The favored mechanism is heating by the Active Galactic Nucleus (AGN) harbored in the cluster’s central galaxy. AGN jets blow radio bubbles and inject copious amounts of energy into the ICM in the form of relativistic plasma as well as mechanical energy. Signs of AGN activity (e.g., X-ray cavities filled by the radio bubbles) are seen in most cluster cores. However, the precise mechanism by which the AGN heats the ICM is still unclear. Complicating the picture is the recent finding that the most powerful AGN outbursts observed in the X-rays appear to deposit most of their energy at a certain distance from the center, outside the region of rapid cooling. A spectacular example is the Ophiuchus cluster, where a giant (~500 kpc across) cavity in the X-ray gas has been created by the blast of an extremely energetic AGN outburst occurred a few hundred million years ago. In this talk, I will present new deep radio and X-ray images of this exceptional fossil of the most energetic AGN outburst seen in any galaxy cluster. It may be an early example of a new class of sources to be uncovered by deeper radio and X-ray observations of galaxy clusters.