A confirmed location in the Galactic halo for the high-velocity cloud 'chain A'

The high-velocity clouds of atomic hydrogen, discovered about 35 years ago( 1,2), have velocities inconsistent with simple Galactic rotation models tha t generally fit the stars and gas in the Milky Way disk. Their origins and role in Galactic evolution remain poorly understood(3), largely for lack of information on their distances. The high-velocity clouds might result from gas blown from the Milky Way disk into the halo by supernovae(4,5), in whi ch case they would enrich the Galaxy with heavy elements as they fall back onto the disk. Alternatively, they may consist of metal-poor gas-remnants o f the era of galaxy formation(2,6-8), accreted by the Galaxy and reducing i ts metal abundance. Or they might be truly extragalactic objects in the Loc al Group of galaxies(7-9). Here we report a firm distance bracket for a lar ge high-velocity cloud, chain A, which places it in the Milky Way halo (2.5 to 7 kiloparsecs above the Galactic plane), rather than at an extragalacti c distance, and constrains its gas mass to between 10(5) and 2x10(6) solar masses.