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Arif Babul Colloquium Abstract (Feb 14, 2018)

Galaxy Clusters as Cosmological Probes?

Clusters of galaxies are the largest, most massive gravitationally bound objects in the Universe. They are also the most recent of the cosmic objects to form. According to the currently accepted models of cosmic structure formation, the number density distribution of these systems is very sensitive to the parameters describing the large-scale geometry and the expansion history of the universe. For this reason, galaxy clusters are regarded as important cosmological probes. However, to useĀ  clusters as precision probes of the cosmological parameters, we need to be able to "weigh them". To do so, and do so properly is challenging. Here I will describe our effort to get a handle on the various systematics and biases that can influence the outcome, and present our mass measurements for 50 galaxy clusters comprising the "Canadian Cluster Comparison Project" or CCCP sample and the LoCuSS Cluster Sample. Using clusters as cosmological probes, however, requires many more than 50-odd clusters with known masses but this is not feasible at the present. I will discuss our ongoing effort to identify and calibrate "easy-to-observe" proxies for the mass, focusing on the clusters' Compton Y-parameter. One remarkable upshot from this work is that cosmology suggested by clusters is in tension with Planck CMB results.

Technical Talk: AGN Feedback In Groups and Clusters Of Galaxies

NOTE: This talk will be given onTuesday, February 13, at 1 p.m. in Room 505 in the Goldwater Center

There is a broad consensus that AGN feedback in galaxy groups and clusters is essential for preventing catastrophic cooling. However, in spite of years of intense study, there remains considerable uncertainly about some of the basic physics processes involved, like how mass flows onto the central supermassive black hole and how narrow jets are able to heat the diffuse gas nearly isotropically. Here, I will offer new insights based on analytic arguments as well as our recent high-resolution numerical simulation studies. I will argue that gas flow onto the black hole is dominated by infall of clouds condensing out of the thermally unstable intracluster medium - and not Bondi accretion, as is generally assumed. I will also try to convince you that every so often, the cold clouds will cause the jet emanating from the black holes to slew and that these changes direction are key to efficient isotropic heating.