Cold fronts by merging of shocks

Abstract

Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of galaxies. We propose that some CFs are relics of merging between two shocks propagating in the same direction. Such shock mergers typically result in a quasi-spherical, a factor of ≈1.4–2.7 discontinuity in density and in temperature. These CFs may be found as far out as the virial shock, unlike what is expected in other CF formation models. As a demonstration of this effect, we use 1D simulations of clusters and show that shock-induced CFs form when perturbations such as explosions or mergers occur near the cluster's centre. Perturbations at a cluster's core induce periodic merging between the virial shock and outgoing secondary shocks. These collisions yield a distinctive, concentric, geometric sequence of CFs which trace the expansion of the virial shock.