Direct Simulation Monte Carlo for astrophysical flows: II. Ram pressure dynamics

TL;DR

This paper employs the DSMC method combined with n-body simulations to explore ram pressure effects on galaxy-ICM interactions, revealing new insights into shock dynamics, gas heating, and morphological features like rings and spokes.

Contribution

It introduces a gas kinetic approach to astrophysical flows, explicitly modeling interfaces and conservation laws, providing new physical insights into ram pressure stripping phenomena.

Findings

Shock and backward wave formation at ICM-ISM contact.

Hot post-shock gas flows around the galaxy, causing heating and ablation.

Kelvin-Helmholtz instability leads to ring and spoke structures.

Abstract

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.