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Molecular clouds and galactic spiral structureTwo large-scale 2.6 mm CO surveys of the galactic plane, one in the first quadrant (l = 12 to 60 deg, b = -1 to +1 deg), the other in the second (l = 105 to 139 deg, b = -3 to +3 deg), have provided evidence that, contrary to previous findings, molecular clouds constitute a highly specific tracer of spiral structure. Molecular counterparts of five of the classical 21-cm spiral arms have been identified: the Perseus arm, the local arm (including Lindblad's local expanding ring), the Sagittarius arm, the Scutum arm, and the 4-kpc arm. The region between the local arm and the Perseus arm is apparently devoid of molecular clouds, and the interarm regions of the inner Galaxy appear largely so. CO spiral structure implies that the mean lifetime of molecular clouds cannot be greater than 100 million years, the time required for interstellar matter to cross a spiral arm. Conservation of mass then sets a limit on the fraction of the interstellar medium in the form of molecular clouds: it cannot exceed one-half at any distance from the galactic center in the range 4-12 kpc.
Document ID
19800066230
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Cohen, R. S.
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Cong, H.
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Dame, T. M.
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Thaddeus, P.
(NASA Goddard Institute for Space Studies; Columbia University New York, N.Y., United States)
Date Acquired
August 10, 2013
Publication Date
July 15, 1980
Publication Information
Publication: Astrophysical Journal
Subject Category
Astrophysics
Accession Number
80A50400
Distribution Limits
Public
Copyright
Other

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