Discovery of a Population of H I Clouds in the Galactic Halo (Accepted for Publication in the Astrophysical Journal Letters)

A population of discrete H I 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 tangent points throughout the first longitude quadrant and at several locations in the fourth quadrant. Their velocities follow Galactic rotation. A group of clouds more than 500 pc below the plane near l = 29 was studied in detail. In the median, the 38 clouds have a peak NHI of a few 10 cm, a diameter of a few tens of pc, an H I density of a few tenths cm, and an H I mass of 50 M⊙, with a considerable range about the median. Some halo clouds have line widths so narrow that their temperature must be < 1000 K. Some appear to have a core-halo velocity structure. As much as half the mass of the neutral halo may be in clouds. Subject headings: Galaxy:halo,structure – ISM:clouds,structure – Radio Lines:ISM