Dr. Greg Bryan
Columbia
University of Florida Astronomy Colloquium - March 4th, 2009
Two Puzzles Posed by X-ray Clusters
Cluster galaxies swim in a sea of 100 million degree gas. This gas, identified by its X-ray emission in the 1970's, was the third of the three primary components of clusters to be discovered (dark matter being first identified by Zwicky in the 30's). Because clusters represent huge mass agglomerations and can be seen to large distances, they can be used to map the universe and constrain dark energy. To do so we need to connect cluster observables, such as X-ray luminosity, with the underlying cluster mass, the quantity predicted by theory. However, when we look closely at the structure of the hot gas, we find that robust predictions are fettered by two long-standing puzzles. The first is: why are small X-ray groups so dim (or put more theoretically, why is the entropy of their gas so large)? The second puzzle is the so-called cooling-flow problem: why do some clusters show evidence for cooling gas, but then have no, or little, star formation? Data from the Chandra X-ray telescope has convincingly provided the solution to the second problem: energy input from AGN jets quenches cooling, although precisely how this AGN thermostat works is still not well understood. Simplicity suggests and most researchers think that AGN should also solve the first problem, but I will argue that the correct answer is more closely tied to basic atomic properties of the gas than to black-hole physics.