Molecular clouds and galactic spiral structure
R. S. Cohen, H. Cong, T. M. Dame, P. Thaddeus;
ApJ, 1980, 239, L53
ABSTRACT:Two 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.
KEYWORDS: galactic structure, interstellar matter, milky way galaxy, molecular gases, carbon monoxide, galactic evolution
PERSOKEY:co, milky way, h2, ,
CODE: cohen80