A massive cloud of cold atomic hydrogen in the outer Galaxy

A large fraction of the mass of the interstellar medium in our Galaxy is in the form of warm (10(3)-10(4) K) and cool (50-100 K) atomic hydrogen (H I) gas(1). Cold (10-30 K) regions are thought to be dominated by dense clouds of molecular hydrogen(2). Cold H I is difficult to observe, and therefore our knowledge of its abundance and distribution in the interstellar medium is poor. The few known clouds of cold H I are much smaller in size and mass than typical molecular clouds 3-5. Here we report the discovery that the H I supershell GSH139-03-69 is very cold (10 K). It is about 2 kiloparsecs i n size and as massive as the largest molecular complexes(6). The existence of such an immense structure composed of cold atomic hydrogen in the inters tellar medium runs counter to the prevailing view that cold gas resides alm ost exclusively in clouds dominated by molecular hydrogen.