Extra-planar H Ι in the inner Milky Way

The extra-planar HI in the inner parts of the Milky Way has been discovered to contain numerous cloud-like structures when observed in the 21 cm line with the Green Bank Telescope. 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 M_☉ of HI. Spectra of many of the halo clouds show evidence of coexisting cool and warm HI phases. A preliminary high-resolution study of one of the clouds suggests that it consists of a di®use envelope and a few dense cores, with a peak N_{H I} reaching 4 X 10²⁰ cm^-2. The clouds are often organized into larger structures, one example of which was discovered near ζ = 35° rising higher than 2 kpc above the Galactic plane. New observations should answer some fundamental questions about the nature of these clouds.