MAGNETIZED GAS IN THE SMITH HIGH VELOCITY CLOUD

We report the first detection of magnetic fields associated with the Smith High Velocity Cloud. We use a catalog of Faraday rotation measures toward extragalactic radio sources behind the Smith Cloud, new Hi observations from the Robert C. Byrd 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 (RM) of approximate to 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 approximate to 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 H I 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 (approximate to 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.