Abstract

The Galactic halo contains a complex ecosystem of multiphase intermediate-velocity and high-velocity gas clouds whose origin has defied clear explanation. They are generally believed to be involved in a Galaxy-wide recycling process, either through an accretion flow or a large-scale fountain flow, or both. In this talk I will present my work on how the magnetic field in the halo affects these processes using magnetohydrodynamic (MHD) simulations. The magnetic field becomes ‘draped’ around the clouds as they move through the halo and this field suppresses hydrodynamic instabilities at the cloud-halo interface. This has widespread implications for gas accretion. The suppression of instabilities prolongs the survival of High-Velocity Clouds (HVCs) so that they are more likely to reach the disk. It also suppresses the mixing of cloud-halo material in the wakes of clouds ejected by the Galactic fountain. This leads to less condensation of cold gas that can be accreted through the fountain process. In addition, the draping of the magnetic field around clouds means that observational constraints on magnetic fields around HVCs can be used to roughly infer their distances. This is useful to constrain their masses and thereby how important they are as a source of accretion.