Why Do Only Some Galaxy Clusters Have Cool Cores?

Abstract

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 paper, cosmological N-body + Eulerian hydrodynamic simulations, including radiative cooling and heating, are used to address this question as we examine the formation and evolution of cool core (CC) and non-cool core (NCC) clusters. These adaptive mesh refinement simulations produce both CC and NCC clusters in the same volume. They have a peak resolution of 15.6 h^{-1} kpc within a (256 h^{-1} Mpc)^3 box. Our simulations suggest that there are important evolutionary differences between CC clusters and their 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 temperatures and hardness ratios beyond the cores, between the distribution of masses, and between their supercluster environs. It also appears that CC clusters are no closer to hydrostatic equilibrium than NCC clusters, an issue important for precision cosmology measurements.Comment: 17 emulateapj pages, 17 figures, replaced with version accepted to Ap

Cosmology and astrophysics from relaxed galaxy clusters – I. Sample selection

“…to the range spanned by the Planck clusters) increases its undisturbed fraction to 26 per cent, reducing the evidence of a trend with redshift, while not significantly changing the picture for the peaky fraction. Both the absolute value of the SZ peaky fraction (14 per cent overall) and its constant behavior with redshift are consistent with the predictions of hydrodynamical simulations (Burns et al 2008, Planelles & Quilis 2009). However, the same simulations predict a decreasing cool-core fraction with cluster mass, which contradicts the increasing fraction of peaky clusters with temperature observed for the SZ sample.…”

“…There are several known examples of cool cores being destroyed by ram pressure stripping as they oscillate (slosh) about the bottom of the cluster potential following a merger (Markevitch et al 2000, Mazzotta et al 2001, Million et al 2010, Ehlert et al 2011, Ichinohe et al 2014, a process also observed in hydrodynamic simulations (e.g. Burns et al 2008, ZuHone et al 2011. Hence a possible explanation is that mergers with the necessary mass ratio and impact parameter to destroy a hosted cool core via sloshing are relatively less common for the most massive clusters, despite these clusters having a larger merger rate overall; this would be qualitatively consistent with the larger undisturbed fraction we observe for the most massive clusters.…”

Rhapsody-G simulations I: the cool cores, hot gas and stellar content of massive galaxy clusters

“…The NCC systems follow the observational relation reasonably well. The fossil cluster and the CC systems however show a very large central gas fraction, in many cases even above the universal gas fraction (similar to the results of Burns et al 2008). This result is clearly in tension with the results of Mantz et al (2014), and reflects the high electron densities and very low central entropies we have seen for the CC systems above.…”

A pan-chromatic view of the galaxy cluster XMMU J1230.3+1339 atz= 0.975

“…However, once including the continuing mass accretion the cluster growth rate since z 1 is much faster. The simulations of Burns et al (2008) predict average mass growth rates of 5−10% per Gyr at late times and 30−50% per Gyr during active mass assembly periods around z ∼ 1. As a benchmark, the time evolution of the largest local ∼10 15 M halo in the Millennium-II simulation (Boylan-Kolchin et al 2009) exhibits a total mass increase since z ∼ 1 by more than a factor of 5.…”

“…This scenario is consistent with the emerging picture from simulations on the different formation histories of NCC and CC clusters. The simulations of Burns et al (2008) showed that Non-Cool-Core clusters are characterized by an increased mass accretion rate ( > ∼ 50% per Gyr) in early epochs (z > ∼ 0.8), during which nascent cool cores are destroyed by major merger events which also set the conditions to prevent cooling at later epochs.…”