Direct Simulation Monte Carlo for astrophysical flows: II. Ram pressure
  dynamics
  We use the Direct Simulation Monte Carlo (DSMC) method combined with an
n-body code to study the dynamics of the interaction between a gas-rich spiral
galaxy and intracluster or intragroup medium, often known as the ram pressure
scenario. The advantage of this gas kinetic approach over traditional
hydrodynamics is explicit treatment of the interface between the hot and cold,
dense and rarefied media typical of astrophysical flows and the explicit
conservation of energy and momentum and the interface. This approach yields
some new physical insight. Owing to the shock and backward wave that forms at
the point ICM--ISM contact, ICM gas is compressed, heated and slowed. The shock
morphology is Mach-disk-like. In the outer galaxy, the hot turbulent post-shock
gas flows around the galaxy disk, while heating and ablating the initially cool
disk gas. The outer gas and angular momentum are lost to the flow. In the inner
galaxy, the hot gas pressurizes the neutral ISM gas causing a strong two-phase
instability. As a result, the momentum of the wind is no longer impulsively
communicated to the cold gas as assumed in the Gunn-Gott (1972) formula, but
oozes through the porous disk, transferring its linear momentum to the disk en
masse. The escaping gas mixture has a net positive angular momentum and forms a
slowly rotating sheath. The shear flow caused by the post-shock ICM flowing
through the porous multiphase ISM creates a strong Kelvin-Helmholtz instability
in the disk that results in Cartwheel-like ring and spoke morphology.
