Magnetic fields in the Galactic halo restrict fountain-driven recycling and accretion

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. We examine the evolution of these clouds in light of recent claims that they may trigger condensation of gas from the Galactic corona as they move through it. We measure condensation along a cloud's wake, with and without the presence of an ambient magnetic field, using two- (2D) and three-dimensional (3D), high-resolution simulations. We find that 3D simulations are essential to correctly capture the condensation in all cases. Magnetic fields significantly inhibit condensation in the wake of clouds at $t \gtrsim 25$ Myr, preventing the sharp upturn in cold gas mass seen in previous non-magnetic studies. The magnetic field suppresses the Kelvin-Helmholtz instability responsible for the ablation and consequent mixing of a cloud with halo gas which drives the condensation. This effect is universal across different cloud properties (density, metallicity, velocity) and magnetic field properties (strength and orientation). Simple convergence tests demonstrate that resolving the gas on progressively smaller scales leads to even less condensation. While condensation still occurs in all cases, our results show that an ambient magnetic field drastically lowers the efficiency of fountain-driven accretion and likely also accretion from condensation around high-velocity clouds. These lower specific accretion rates are in better agreement with observational constraints compared to 3D, non-magnetic simulations.