Publications:

List of primary publications. Click here.

Some Recent publications:

1. Tim O'Hara's last thesis paper focused on a structural analysis of a sample of 70 clusters taken from the Chandra archive. The analysis reveals that these systems evolve with redshift but not in a manner consistent with the self-similar expectation. A consistent explanation is that there is a redshift trend for varying ICM mass fraction. Some mix of radiative cooling, AGN feedback and early feedback due to star formation at high redshift is likely to be able to provide evolution consistent with that observed, but at present the theoretical work remains to be done.
2. We have completed a systematic analysis of the radio galaxy population using the NVSS and FIRST surveys together with a sample of 600 clusters selected in ROSAT all sky survey. We measure the radial distribution of these sources, finding that they are significantly more centrally concentrated than the galaxy populations. In addition, we find that the probability of a galaxy hosting an active radio galaxy in a cluster is about twice as high as in the field. Together, these suggest that the cluster environment- in particular the centers of clusters- are enabling environments for the physical processes driving the AGN radio emission. We explore the impact that these sources could have on the central intracluster medium energy budget, and we explore the effects of these sources on SZE selection of galaxy clusters. Our measurements indicate that the radio luminosity from AGN in clusters is typically comparable to the SZE "luminosity" for low frequency surveys.
3. We have explored the sources of scatter in cluster scaling relations for a large sample of nearby clusters. Indications are that the dominant contributor to scatter is the central cool core region. We examine whether clusters having recently undergone mergers (as indicated by morphological merger indicators) exhibit more scatter and find that this is not the case.
4. We have a completed a joint 2MASS and X-ray analysis of the most luminous cluster galaxies (BCGs) and the intracluster light in an ensemble of 93 groups and clusters. Our analysis shows that BCG luminosity increases mildly with cluster mass, and that the fractional contribution from the BCG to the cluster light falls rapidly with cluster mass. We examine BCG formation and evolution scenarios and suggest that BCG's grow primarily through mergers with luminous galaxies- often the BCG's from the less massive groups and clusters merging with the system. We use a merger tree model together with our observations to examine the intracluster light, showing that it is an increasing fraction of cluster light that reaches approximately 50% in massive clusters. The ICL helps in sorting out some problems with the high iron content of the intracluster medium and in bringing the observed cold baryon fraction into better agreement with estimates from hydrodynamical structure formation scenarios. The baryon fraction including the ICL is in good agreement with WMAP CMB anisotropy constraints. Read more.