The Observational Consequences of Merging Clusters of Galaxies

Abstract

We present an observational analysis of the numerical simulations of galaxy custer mergers. We identify several observational signatures of recent merger activity and quantitatively assess the uncertainty introduced into cluster mass estimates when invoking the commonly held assumptions of hydrostatic equilibrium, virial equilibrium, spherical symmetry, and isothermality. We find that mergers result in multiple X-ray peaks, long-lived elongation of the X-ray emission, as well as isophotal twisting and centroid shifting to a degree consistent with recent observations. We also find an enlargement of the X-ray core relative to the dark matter core. Mergers result in nonisothermal clusters exhibiting observable inhomogeneities in the emission-weighted X-ray temperature of several keV on linear scales of less than 0.5 Mpc. The resulting gasdynamics are extremely complex, and we present an example of what might be observed by a high resolution X-ray spectograph. We further speculate that the gas dynamics, via shocks, bulk flows, and turbulence, play an important role in the evolution of cluster galaxies and associated radio sources, particularly wide-angle tailed (WAT) sources and radio halos. We find that X-ray based cluster mass estimates made under equilibrium assumptions can be uncertain by 50% or more in the first 2 Gyrs after a merger and by up to 25% after 2 Gyrs depending on the details of the analysis and projection effects. Uncertainties can be considerably larger if the temperature is not well constrained. Similar uncertainties are observed in the X-ray derived baryon mass fractions. Virial mass estimates are typically overestimated because the observed one-dimensional velocity dispersion can be severely contaminated by the infall velocity of the subcluster.