The impact of mergers on relaxed X-ray clusters - I. Dynamical evolution and emergent transient structures

Abstract

We report on the analysis of a suite of SPH simulations (incorporating cooling and star formation) of mergers involving idealised X-ray clusters whose initial conditions resemble relaxed clusters with cool compact cores observed by Chandra and XMM. The simulations sample the most interesting, theoretically plausible, range of impact parameters and progenitor mass ratios. We find that all mergers evolve via a common progression. We illustrate this progression in the projected gas density, X-ray surface brightness, SZ, temperature, and gas entropy maps. Several different classes of transient ``cold front''-like features can arise over the course of a merger. We find that all of these classes are present in Chandra and XMM observations of merging systems and propose a naming scheme for these features: ``comet-like'' tails, bridges, plumes, streams and edges. In none of the cases considered do the initial cool compact cores of the primary and the secondary get destroyed during the course of the mergers. We quantify the evolving morphology of our mergers using centroid variance, power ratios and offset between the X-ray and the projected mass maps. We find that the centroid variance best captures the dynamical state of the cluster. Placing the system at z=0.1, we find that all easily identified observable traces of the secondary disappear from a simulated 50 ks Chandra image following the second pericentric passage. The system, however, takes approximately 2 additional Gyrs to relax and virialize. Temperature fluctuations at the level of 20% can persist in the final systems well past the point of virialization, suggesting that that the existence of temperature fluctuations, in and of themselves, do not necessarily indicate a disturbed or unrelaxed system.