Discovery of a Population of H [CSC]i[/CSC] Clouds in the Galactic Halo

Abstract

A population of discrete H I clouds in the halo of the inner Galaxy has been discovered in 21 cm 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^deg was studied in detail. In the median, the 38 clouds have a peak N_HI of a few times 10^19 cm^-2, a diameter of a few tens of parsecs, an H I density of a few tenths cm^-3, and an H I mass of 50 M_solar, with a considerable range about the median. Some halo clouds have line widths so narrow that their temperature must be less than 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.