Michael McDonald

The massive galaxy clusters IDCS J1426.5+3508, which, at z=1.75, is amonst the most distant clusters known. This system has a rich ICM and a well developed population of galaxies. Systems such as this probably collapsed and virialized at z ~ 3, an epoch that will soon be reachable with the SPT-3G survey.

The First Galaxy Clusters

The study of the most distant, high redshift clusters has recently exploded with the success of mm-wave surveys which detect galaxies via the Sunyaev Zel'dovich effect. In particular, the South Pole Telescope has discovered hundreds of new clusters out to a redshift of 1.85, or 10 billion years in the past. These newly-discovered systems are providing our first glimpse of galaxy clusters shortly after they collapsed out of the cosmic web. Future surveys, such as SPT-3G, will find thousands of new galaxy clusters, including the first virialized systems at z > 2.

Our results thus far have indicated that galaxy clusters have evolved self similarly since z ~ 2, with the exception of their cores (r < 100 kpc). The cores of clusters appear to have formed early (z ~ 1.5), and then remained stable in size, temperature, and mass while the rest of the cluster has evolved around it. The metal enrichment of the cluster appears to have happened early (z > 1), while in the core there has been an additional enrichment that happened since z ~ 1.

These and other results are presented in a series of papers: McDonald et al. (2013b, 2014c, 2016b, 2017a)

Three-color broadband image of the central galaxy in the Phoenix cluster. This galaxy is experiencing a massive ~800 Msun/yr starburst, which is most likely being fueled by rapid cooling of the intracluster medium. This may be a short-lived phase of evolution that central galaxies in the most massive clusters go through. Our recent work suggests that the highest mass clusters may go through phases of inefficient feedback, as their AGN transition between radio and quasar mode.

AGN Feedback and Cooling Flows

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, I have been trying to understand the evolution the cooling/feedback balance in a sample of massive, SZ-selected galaxy clusters. 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. At low redshift, we are finding evidence that the most massive clusters can experience inefficient feedback, as their central black holes oscillate between mechanical (radio jets) and radiative (quasar mode) feedback. This leads to wild swings in the star formation rate - nearly four orders of magnitude - for central galaxies in the strongest cool cores.

Hubble Space Telescope image of the central galaxy in a massive galaxy cluster at z > 1. This central galaxy, which was unresolved in ground-based images, appears to be in the late stage of a dry merger. Our latest results suggest that these central, massive galaxies were growing rapidly via both wet and dry mergers at early times.

The Growth of Giant Elliptical Galaxies

The central galaxies in clusters occupy a special place and, as such, experience a different evolution than the rest of the cluster members. These systems are thought to grow primarily via dry (gas-poor) mergers alongside the rest of the cluster, leading to a relationship between the central galaxy mass and the total cluster mass.

Recent work by our work and other groups have shown that there is significant amounts of star formation in the central galaxies of massive clusters at high redshift. This star formation appears to be fueled by gas-rich mergers at early times (z>1) and residual cooling of the ICM ate late times (z<<1). At even earlier times, much of this star formation may be fueled by in situ gas, such as in the famous Spiderweb protocluster. Our group is trying to understand the relative importance of all of these factors (dry/wet mergers, cooling flows, in situ star formation) in assembling the final mass of the most massive galaxies in the Universe.

We are in the midst of conducting a larger HST survey of central galaxies to determine the rate of dry and wet mergers at the centers of cluster over the past 10 billion years of cluster evolution.