AGN Feedback in groups and clusters

The common occurrence and variety of cavities, shocks, and cold fronts in Chandra/XMM-Newton X-ray observations of the hot intergalactic medium (IGM) in groups and clusters, provides direct evidence of the widespread presence of AGN-driven phenomena. Radio observations of these systems provide a complementary view, revealing the presence of associated radio galaxies, with jets, lobes and bubbles. Combining these with dynamical ages of cavities, one can estimate their total mechanical energy and the mean power of the jets. Radio observations at <1 GHz can also throw light on the composition of the jets that transfer energy from the AGN to their surroundings, and the contents of the cavities, for which relativistic electrons, protons, positrons and MHD currents have all been suggested as important constituents. Lower frequency data are crucial for understanding the content of radio sources, since they are most sensitive to the history of AGN activity over timescales > 10^8 yr, while high frequency observations are sensitive to the instantaneous jet power.

Smoothed Chandra X-ray image of the HCG62 group, in which two ~8 kpc cavities are detected. In both cases, our GMRT observation (235 MHz contours on Chandra 0.3-2 keV image) show far more extended emission than previous VLA observations, and evidence of progressive spectral ageing along the jets.

We are involved in a campaign of radio observations of a sample of 18 elliptical-dominated groups, which show structures indicative of AGN interaction in deep Chandra/XMM X-ray images, at 3 GMRT frequencies (230-610 MHz), in collaboration with colleagues at CfA, Bologna and NCRA (HCG62, shown above, is one of them). A similar project, involving 25 rich clusters from the Birzan et al (2004) sample, is under way as well, in collaboration with colleagues at Waterloo, CfA, Astron and NCRA. The targets include Hercules A, Hydra A and Abell 2052.

Low frequency GMRT observations (0.15-1.4 GHz) reveal the extent of AGN activity interacting with the hot IGM in the galaxy group AWM4 (Chandra, 0.3-2 keV, in colour).

Comparison of real and simulated groups

To study the physics of feedback, we are collaborating with Ian McCarthy (Cambridge) to compare our observations with groups in OWLS, a suite of very large (2.7E8 particles) cosmological simulations specifically designed to explore the impact of feedback using a variety of different prescriptions for incorporating supernova-driven winds and feedback from AGN.

A deep Chandra study of high-z groups

The evolution in group properties provides strong diagnostics of the history of feedback. However, studying the hot gas in groups at z>0.3 requires very long observations with Chandra or XMM, and has been achieved only for small samples. We are searching the deepest archival Chandra fields for groups, and studying their gas profiles and X-ray morphology. We will follow up with optical observations of our newly discovered groups.

Researchers: Somak Raychaudhury, Trevor Ponman, Ewan O'Sullivan, Alastair Sanderson, Aurelia Pascut, Matt Lazell