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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 (103–104 K) and cool (50–100 K) atomic hydrogen (H i) gas1. Cold (10–30 K) regions are thought to be dominated by dense clouds of molecular hydrogen2. 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 clouds3,4,5. 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 complexes6. 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.

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Figure 1: The cold H i arc as observed by the Canadian Galactic Plane Survey.
Figure 2: The H i supershell GSH139-03-69.
Figure 3: Evidence for cold H i towards the dark arc from continuum absorption.
Figure 4: On-arc and off-arc H i spectra.


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We thank T. Dame for providing the CO data from ref. 2. The Canadian Galactic Plane Survey (CGPS) is a Canadian project with international partners. The Dominion Radio Astrophysical Observatory is operated as a national facility by the National Research Council of Canada. The CGPS is supported by the Natural Sciences and Engineering Research Council of Canada.

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Correspondence to Lewis B. G. Knee.

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Knee, L., Brunt, C. A massive cloud of cold atomic hydrogen in the outer Galaxy. Nature 412, 308–310 (2001).

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