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