Anatomy of a Merger: A Numerical Model of A754

Abstract

A754 is a well-observed cluster of galaxies (z = 0.054) that exhibits a variety of morphological peculiarities. These include a bar of X-ray emission that is offset significantly from the galaxy distribution, an elongated X-ray surface brightness distribution extending between two distinct peaks in the galaxy distribution, and an extremely nonisothermal and asymmetric intracluster medium temperature morphology. Using these observational constraints, we present a numerical hydro/N-body model of A754 in which two clusters (2.5:1 mass ratio) have merged nearly in the plane of the sky less than 0.5 Gyr ago with an impact parameter of ~120 kpc and an impact velocity of ~2500 km s-1 (roughly the escape velocity of the primary cluster). Our models allow us to identify the origin of A754's peculiar X-ray and temperature morphologies, the underlying hydrodynamical processes that shape them, and their future evolution. We make detailed predictions for future high-resolution X-ray spectroscopic observations (e.g., Astro-E). We discuss general properties of our models that will be characteristic of off-axis mergers. In particular, we find significant nonthermal pressure support within the central region that could bias cluster mass estimates. We find significant angular momentum imparted on the gas distribution in the cluster. We find that mixing of the subcluster gas components is an inefficient process, particularly at large radii. Finally, we find that subsequent dark matter core passages result in an extended relaxation timescale.