The Origin of Cold Fronts in the Cores of Relaxed Galaxy Clusters

Chandra X-ray observations revealed the presence of cold fronts (sharpcontact discontinuities between gas regions with different temperatures anddensities) in the centers of many, if not most, relaxed clusters with coolcores. We use high-resolution simulations of idealized cluster mergers to showthat they are due to sloshing of the cool gas in the central gravitationalpotential, which is easily set off by minor mergers and can persist forgigayears. The only necessary condition is a steep entropy profile, as observedin cooling flow clusters. Even if the infalling subcluster has no gas duringcore passage, the gravitational disturbance sets the main mass peak (gas anddark matter together) in motion relative to the surrounding gas. A rapid changein the direction of motion causes a change in ram pressure, which pushes thecool gas away from the dark matter peak and triggers sloshing. For nonzeroimpact parameters, the cool gas acquires angular momentum, resulting in acharacteristic spiral pattern of cold fronts. There is little visibledisturbance outside the cool core in such a merger. If the subcluster retainsits gas during core passage, the cool central gas of the main cluster is moreeasily decoupled from the dark matter peak. Subsequently, some of that gas, andoften the cool gas from the subcluster, falls back to the center and startssloshing. However, in such a merger, global disturbances are readily visible inX-rays for a long time. We conclude that cold fronts at the centers of relaxedclusters, often spiral or concentric-arc in shape, are probably caused byencounters with small subhalos stripped of all their gas at the early infallstages.Comment: 24 pages (emulateapj), 23 figures. Minor changes to match accepted versio