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 ago1,2, have velocities inconsistent with simple Galactic rotation models that generally fit the stars and gas in the Milky Way disk. Their origins and role in Galactic evolution remain poorly understood3, 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 supernovae4,5, in which 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 of the era of galaxy formation2,6,7,8, accreted by the Galaxy and reducing its metal abundance. Or they might be truly extragalactic objects in the Local Group of galaxies7,8,9. Here we report a firm distance bracket for a large 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 extragalactic distance, and constrains its gas mass to between 105 and 2×106 solar masses.