The talks are in the same order as the Program Schedule.
Vrtilek, Jan M. - X-ray and radio observations of galaxy groups: the history of AGN heating
Burns, Jack - Why Do Only Some Galaxy Clusters Have Cool Cores?
Brian McNamara (University of Waterloo)
For more than 25 years, it was thought that dense gas in the cores of clusters cooled at rates of tens to hundreds of solar masses per year, and condensed in a largely unseen form. This perception changed dramatically when X-ray observations of galaxy clusters by the Chandra and XMM-Newton observatories failed to find gas cooling below X-ray temperatures at the expected rates. At about the same time, images from the Chandra observatory revealed giant cavities and shock fronts that provide a direct and reliable means of measuring the energy injected into hot atmospheres by active galactic nuclei (AGN). Average AGN powers are near those required to suppress cooling in isolated giant elliptical galaxies, and in larger systems up to the richest galaxy clusters. This coincidence suggests that heating and cooling are coupled by feedback, which suppresses star formation and the growth of luminous galaxies. I will discuss the broader implication of these results for several fundamental astrophysical problems including the growth of bulges and supermassive black holes, the excess entropy in hot halos, and baryon overcooling in the universe.
Jan M. Vrtilek (Center for Astrophysics) , S. Giacintucci (CfA and INAF), E. O'Sullivan (CfA), L. David (CfA), S. Raychaudhury (U. Birmingham), T. Ponman (U. Brimingham), W. Forman (CfA), C. Jones (CfA)
The central regions of galaxy groups and clusters are frequently occupied by massive elliptical galaxies with powerful radio sources which strongly interact with the X-ray emitting gas. Recent studies of such interactions have focused mostly on massive clusters, but there is growing interest in extending investigations to groups, which are the locus of most galaxies in the present-day Universe, and which may be more affected by AGN outbursts owing to shallow group potentials and may make radio/ICM interaction more apparent because of the low-pressure group environment. We present observations of about 15 groups, for all of which excellent X-ray data are in hand, and for which we are obtaining low frequency radio data, principally with the GMRT. Radio observations at multiple (especially low) frequencies show the radiative aging of electron populations and can reveal the radio counterparts to X-ray "ghost" cavities that contain old/low-energy electron populations. We present initial results from this observing program that illuminate the mechanisms, distribution, and timescales of energy injection.
Jack Burns (University of Colorado, Boulder)
Flux-limited X-ray samples indicate that about half of rich galaxy clusters have cool cores. Why do only some clusters have cool cores while others do not? In this talk, I will present cosmological adaptive mesh N-body + Eulerian hydrodynamic simulations, including radiative cooling and heating, along with Chandra and ROSAT observations to address this question. Our simulations suggest that there are important evolutionary differences between cool core (CC) clusters and their non-cool core (NCC) counterparts. Many of the numerical CC clusters accreted mass more slowly over time and grew enhanced cool cores via hierarchical mergers; when late major mergers occurred, the CC's survived the collisions. By contrast, NCC clusters experienced major mergers early in their evolution that destroyed embryonic cool cores and produced conditions that prevented CC re-formation. As a result, our simulations predict observationally testable distinctions in the properties of CC and NCC beyond the core regions in clusters. In particular, we find differences between CC versus NCC clusters in the shapes of X-ray surface brightness profiles, between the temperature profiles and hardness ratios beyond the cores, and between the distribution of masses. It also appears that CC clusters are no closer to hydrostatic equilibrium than NCC clusters, an issue important for precision cosmology measurements. I will compare these predictions with data from Chandra and ROSAT.