Molecular clouds and galactic spiral structure

Two large-scale 2.6 mm CO surveys of the galactic plane, one in the first quadrant (l=12/sup 0/ to 60/sup 0/, b=-1/sup 0/ to +1/sup 0/), the other in the second (l=105/sup 0/ to 139/sup 0/, b=-3/sup 0/ to +3/sup 0/), 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 1 x 10/sup 8/ 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.