X-ray emission from galaxies, groups and clusters

(Hardcastle, Brassington, Goodger)

The majority of the baryonic matter in the Universe is in the form of hot, diffuse X-ray-emitting gas comprising the intergalactic medium in galaxy groups and clusters. X-ray-emitting hot gas is also an important component of the interstellar medium of elliptical galaxies. The thermal X-ray emission from galaxies, galaxy groups and clusters contains the signatures of important evolutionary processes such as mergers, the activity of active galactic nuclei (AGN), galaxy interactions and tidal stripping. As a tracer of the underlying dark matter distribution, the X-ray emission also provides a method of testing cosmological models of structure formation.

The impact of AGN activity on the hot, X-ray emitting gas in galaxies, groups and clusters is now understood to be one of the major uncertainties in our understanding our how the Universe has evolved to its present state. In particular, the outbursts from radio-loud AGN appear to be a crucial ingredient for models of galaxy evolution and of the co-evolution of galaxy bulges and black holes, as well as for understanding the observed properties of the intracluster medium. It is also essential that the effects of AGN outbursts on scaling relations between X-ray observables and cluster mass are understood in order to test structure formation models and to use galaxy clusters as cosmological probes.

The current generation of X-ray observatories (XMM-Newton and Chandra) has provided much of the evidence to support this emerging picture of an important feedback role for radio-loud AGN. Our work combines detailed X-ray imaging and spectroscopy with low-frequency radio imaging using the VLA, GMRT and ATCA to investigate the energy transfer mechanisms and feedback roles of different radio-loud AGN populations.

Current projects include:

• Detailed radio/X-ray studies of individual galaxies in which radio-source outbursts are having a dramatic impact on the ISM. Our recent work on strong shocks in elliptical galaxies suggests that the energy input from small (young?) radio galaxies on kpc scales may be an important process for the elliptical galaxy population as a whole.
• X-ray environmental studies for samples of radio-loud AGN. We are interested in constraining the environments of different radio-loud AGN populations (e.g. Fanaroff-Riley type I and II sources; low vs. high excitation sources) and looking for direct evidence of heating effects, so as to understand the energetics, energy transfer mechanisms, and feedback roles of these different populations.
• X-ray studies of galaxy group and cluster samples. We are interested in understanding how the gas physics of galaxy groups and clusters (e.g. the effects of AGN activity, cluster mergers, galaxy interactions) influence the scaling relations between X-ray observables and cluster mass. In particular, our recent work has identified statistical signatures of radio-galaxy impact in galaxy groups. We are collaborating on projects to map the gas and dark matter distributions of the nearby cluster population, as well as projects focused specifically on radio-source impact.
• Radio studies of galaxy group and cluster samples. By investigating the radio content and characteristics of X-ray selected group and cluster samples, and their relationship to cluster gas properties (as mapped in detail with XMM-Newton and Chandra) we aim to determine the origin of observed radio-source effects on the group luminosity-temperature relation and more generally to constrain the role of AGN outbursts in group/cluster evolution.
• Hydrodynamical modelling of radio-galaxy/environment interactions. We are investigating how different energy injection histories affect the X-ray properties of galaxy groups and clusters.