  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.
