Prof. Michael A McDonald

One of the great mysteries in astronomy today is why >90% of the baryons in the Universe are locked up in hot, diffuse gas with short cooling times. This problem manifests as a serious disagreement between the predicted luminosity function and the observed one - there are simply not enough stars being formed in the most massive and least massive galaxies. At the low-mass end, this problem is known as the "missing satellite problem", while at the high-mass end it is known as the "cooling flow problem".

One place where this disagreement can be studied in detail is the cores of massive galaxy clusters. Here, the hot intracluster medium is dense enough to retain imprints of past energetic events, maintaining fossil records of possible physical processes that could prevent the overcooling of the hot gas. By necessity, this work requires use of a wide variety of data, including X-ray, UV, optical, IR, sub-mm, and radio observations in order to track gas cooling from the hot (>107K) to cold (~10K) phases.

Recently, in collaboration with the South Pole Telescope (SPT) team, Professor McDonald has been trying to understand the evolution the cooling/feedback balance in a sample of massive, SZ-selected galaxy clusters. This nearly redshift independent sample is proving to be invaluable for studying the evolution of the hot, intracluster medium. Early results suggest that cooling may have been more favorable at earlier times (z~1), but over the past 8 Gyr cooling and feedback have been remarkably well-matched.

For his Ph.D. thesis, McDonald used the Maryland-Magellan Tunable Filter to examine heating and cooling processes in galaxy groups and clusters, primarily by looking for extended H-alpha emission. The exquisite resolution (DIQ ~ 0.5") of these data allows us to resolve structure which was previously unknown, such as the intertwined filaments in Abell 1795 (left). By combining the emission-line data with existing Chandra and GALEX observations, as well as new HST observations in the far-UV, we shed new light on these mysterious structures.

For his M.Sc. thesis, McDonald conducted a near-IR (H-band) survey of the Virgo cluster with Stephane Courteau (Queen's University) and Brent Tully (University of Hawaii). We obtained optical and H-band photometry for a complete sample of 293 Virgo cluster galaxies, presented here, which was used to perform a detailed analysis on the structural properties of these galaxies via bulge-disk decompositions.