Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

The Milky Way's halo contains clouds of neutral hydrogen with high radial v elocities which do not follow the general rotational motion of the Galaxy(1 ). Few distances to these high-velocity clouds are known(2,3), so even gros s properties such as total mass are hard to determine. As a consequence, th ere is no generally accepted theory regarding their origin, One idea(4,5) i s that they result from gas that has cooled after being ejected from the Ga laxy through fountain-like flows powered by supernovae; another is that the y are composed of gas, poor in heavy elements, which is falling onto the di sk of the Milky Way from intergalactic space(6,7). The presence of molecula r hydrogen, whose formation generally requires the presence of dust (and th erefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen ab sorption in a high-velocity cloud along the hue of sight to the Large Magel lanic Cloud. We also derive for the same cloud an iron abundance which is h alf of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.