Why Do Only Some Galaxy Clusters Have Cool Cores?
Jack Burns (University of Colorado, Boulder)
Flux-limited X-ray samples indicate that about half of rich galaxy clusters have cool cores. Why do only some clusters have cool cores while others do not? In this talk, I will present cosmological adaptive mesh N-body + Eulerian hydrodynamic simulations, including radiative cooling and heating, along with Chandra and ROSAT observations to address this question. Our simulations suggest that there are important evolutionary differences between cool core (CC) clusters and their non-cool core (NCC) counterparts. Many of the numerical CC clusters accreted mass more slowly over time and grew enhanced cool cores via hierarchical mergers; when late major mergers occurred, the CC's survived the collisions. By contrast, NCC clusters experienced major mergers early in their evolution that destroyed embryonic cool cores and produced conditions that prevented CC re-formation. As a result, our simulations predict observationally testable distinctions in the properties of CC and NCC beyond the core regions in clusters. In particular, we find differences between CC versus NCC clusters in the shapes of X-ray surface brightness profiles, between the temperature profiles and hardness ratios beyond the cores, and between the distribution of masses. It also appears that CC clusters are no closer to hydrostatic equilibrium than NCC clusters, an issue important for precision cosmology measurements. I will compare these predictions with data from Chandra and ROSAT.