Discovery of a Population of HI Clouds in the Galactic Halo

A population of discrete HI clouds in the halo of the inner Galaxy has been discovered in 21cm observations made with the Green Bank Telescope. The halo clouds are seen up to 1.5 kpc from the Galactic plane at many longitudes. Their velocities follow Galactic rotation. A group of clouds chosen for detailed study are found to have, in the median, a peak NHI of 2x10^19, a diameter of a few tens of pc, a density of a few tenths cm^-3, and a mass in HI of 50 solar masses. Some halo clouds have narrow lines implying that their temperature must be <1000 K; some have a core-halo spectral structure. As much as half of the mass of the neutral halo may reside in these discrete clouds. They may have been formed in a Galactic fountain and are now returning to the disk.