The origin of cold fronts in the cores of relaxed galaxy clusters

Abstract

Chandra X-ray observations revealed the presence of cold fronts (sharp contact discontinuities between gas regions with different temperatures and densities) in the centers of many, if not most, relaxed clusters with cool cores. We use high-resolution simulations of idealized cluster mergers to show that they are due to sloshing of the cool gas in the central gravitational potential, which is easily set off by minor mergers and can persist for gigayears. The only necessary condition is a steep entropy profile, as observed in cooling flow clusters. Even if the infalling subcluster has no gas during core passage, the gravitational disturbance sets the main mass peak (gas and dark matter together) in motion relative to the surrounding gas. A rapid change in the direction of motion causes a change in ram pressure, which pushes the cool gas away from the dark matter peak and triggers sloshing. For nonzero impact parameters, the cool gas acquires angular momentum, resulting in a characteristic spiral pattern of cold fronts. There is little visible disturbance outside the cool core in such a merger. If the subcluster retains its gas during core passage, the cool central gas of the main cluster is more easily decoupled from the dark matter peak. Subsequently, some of that gas, and often the cool gas from the subcluster, falls back to the center and starts sloshing. However, in such a merger, global disturbances are readily visible in X-rays for a long time. We conclude that cold fronts at the centers of relaxed clusters, often spiral or concentric-arc in shape, are probably caused by encounters with small subhalos stripped of all their gas at the early infall stages.