Magnetized High Velocity Clouds in the Galactic Halo: A New Distance Constraint

TL;DR

This paper investigates how magnetic field measurements of high velocity clouds in the Galactic halo can provide new constraints on their distances, cloud evolution, and survival mechanisms.

Contribution

It introduces a model linking magnetic field compression to cloud distance, offering a novel method to estimate HVC distances using observed magnetic fields.

Findings

Magnetic field compression occurs in front of and behind the clouds.

Compressed transverse magnetic fields suppress hydrodynamical instabilities.

Observed magnetic field enhancements can estimate cloud distances from the Galactic plane.

Abstract

High velocity gas that does not conform to Galactic rotation is observed throughout the Galaxy's halo. One component of this gas, HI high velocity clouds (HVCs), have attracted attention since their discovery in the 1960s and remain controversial in terms of their origins, largely due to the lack of reliable distance estimates. The recent discovery of enhanced magnetic fields towards HVCs has encouraged us to explore their connection to cloud evolution, kinematics, and survival as they fall through the magnetized Galactic halo. For a reasonable model for the halo magnetic field, most infalling clouds see transverse rather than radial field lines. We find that significant compression (and thereby amplification) of the ambient magnetic field occurs in front of the cloud and in the tail of material stripped from the cloud. The compressed transverse field attenuates hydrodynamical instabilities. This delays cloud destruction, though not indefinitely. The observed B-field compression is related to the cloud's distance from the Galactic plane. As a result, the observed rotation measure provides useful distance information on a cloud's location.