Extra-planar H I in the Inner Milky Way

Abstract

The extra-planar H I in the inner parts of the Milky Way has been discovered to contain numerous cloud-like structures when observed in the 21cm line with the Green Bank Telescope (GBT). These halo clouds have motions consistent with Galactic rotation and do not seem to be related to the classic high-velocity clouds. They are found to distances >1 kpc from the plane and can contain hundreds of solar masses of H I. Spectra of many of the halo clouds show evidence of coexisting cool and warm H I phases. A preliminary high-resolution study of one of the clouds suggests that it consists of a diffuse envelope and a few dense cores, with a peak H I column density reaching 4 x 10^20 cm^-2. The clouds are often organized into larger structures, one example of which was discovered near 35 degrees Galactic longitude rising higher than 2 kpc above the Galactic plane. New observations should answer some fundamental questions about the nature of these clouds.