title: A massive cloud of cold atomic hydrogen in the outer Galaxy.
abstract: A large fraction of the mass of the interstellar medium in our Galaxy is in the form of warm (103-104 K) and cool (50-100 K) atomic hydrogen (H i) gas. Cold (10-30 K) regions are thought to be dominated by dense clouds of molecular hydrogen. 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. Here we report the discovery that the H i supershell GSH139-03-69 is very cold (10 K). It is about 2 kiloparsecs in size and as massive as the largest molecular complexes. The existence of such an immense structure composed of cold atomic hydrogen in the interstellar medium runs counter to the prevailing view that cold gas resides almost exclusively in clouds dominated by molecular hydrogen.
authors.affiliation: National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton, British Columbia, V2A 6K3, Canada. lewis.knee@nrc.ca
authors.forename: L B
authors.initials: LB
authors.lastname: Knee
authors.forename: C M
authors.initials: CM
authors.lastname: Brunt
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D005118:Extraterrestrial Environment; D006859:Hydrogen / Q000032:analysis*
chemical_list: D006859:Hydrogen