Magnetized gas clouds can survive acceleration by a hot wind

Abstract

We present three-dimensional magnetohydrodynamic simulations of magnetized gas clouds accelerated by hot winds. We initialize gas clouds with tangled internal magnetic fields and show that this field suppresses the disruption of the cloud: rather than mixing into the hot wind as found in hydrodynamic simulations, cloud fragments end up comoving with the external medium and in pressure equilibrium with their surroundings. We also show that a magnetic field in the hot wind enhances the drag force on the cloud by a factor ~(1 + v2A /v2wind), where vA is the Alfvén speed in the wind and vwind measures the relative speed between the cloud and the wind. We apply this result to gas clouds in several astrophysical contexts, including galaxy clusters, galactic winds, the Galactic Centre, and the outskirts of the Galactic halo. Our results can help explain the prevalence of cool gas in galactic winds and galactic haloes, and how this cool gas survives in spite of its interaction with hot wind/halo gas. We also predict that drag forces can lead to a deviation from Keplerian orbits for gas clouds in the galactic center.