Magnetized Gas in the Smith High Velocity Cloud

TL;DR

This study reports the first detection of magnetic fields in the Smith High Velocity Cloud, revealing their potential role in cloud survival and interaction with the Galactic environment through combined radio, HI, and H-alpha observations.

Contribution

It provides the first observational evidence of magnetic fields in the Smith Cloud and estimates their strength, offering new insights into high velocity cloud dynamics.

Findings

Magnetic fields of at least 8 μG detected along a filament.

Correlation between rotation measure enhancements and decelerated H-alpha emission.

Magnetic fields may help high velocity clouds survive during Galactic infall.

Abstract

We report the first detection of magnetic fields associated with the Smith High Velocity Cloud. We use a catalog of Faraday rotation measures towards extragalactic radio sources behind the Smith Cloud, new HI observations from the Green Bank Telescope, and a spectroscopic map of H{\alpha} from the Wisconsin H-Alpha Mapper Northern Sky Survey. There are enhancements in rotation measure of approximately 100 rad m^(-2) which are generally well correlated with decelerated H{\alpha} emission. We estimate a lower limit on the line-of-sight component of the field of approximately 8 {\mu}G along a decelerated filament; this is a lower limit due to our assumptions about the geometry. No RM excess is evident in sightlines dominated by HI or H{\alpha} at the velocity of the Smith Cloud. The smooth H{\alpha} morphology of the emission at the Smith Cloud velocity suggests photoionization by the Galactic ionizing radiation field as the dominant ionization mechanism, while the filamentary morphology and high (approximately 1 Rayleigh) H{\alpha} intensity of the lower-velocity magnetized ionized gas suggests an ionization process associated with shocks due to interaction with the Galactic interstellar medium. The presence of the magnetic field may contribute to the survival of high velocity clouds like the Smith Cloud as they move from the Galactic halo to the disk. We expect these data to provide a test for magnetohydrodynamic simulations of infalling gas.