The role of the halo magnetic field on accretion through high-velocity clouds

Abstract

ABSTRACT High-velocity clouds (HVCs) are believed to be an important source of gas accretion for star formation in the Milky Way. Earlier numerical studies have found that the Galactic magnetic field and radiative cooling strongly affects accretion. However, these effects have not previously been included together in the context of clouds falling through the Milky Way’s gravitational potential. We explore this by simulating an initially stationary cloud falling through the hot hydrostatic corona towards the disc. This represents an HVC that has condensed out of the corona. We include the magnetic field in the corona to examine its effect on accretion of the HVC and its associated cold gas. Remnants of the original cloud survive in all cases, although a strong magnetic field causes it to split into several fragments. We find that mixing of cold and hot gas leads to cooling of coronal gas and an overall growth with time in cold gas mass, despite the low metallicity of the cloud and corona. The role of the magnetic field is to (moderately to severely) suppress the mixing and subsequent cooling, which in turn leads to less accretion compared to when the field is absent. A stronger field leads to less suppression of condensation because it enhances Rayleigh–Taylor instability. However, magnetic tension in a stronger field substantially decelerates condensed cloudlets. These have velocities typically a factor 3–8 below the velocity of the main cloud remnants by the end of the simulation. Some of these cloudlets likely disperse before reaching the disc.