Annu. Rev. Astron. Astrophys. 1982. 20: 517-45 Copyright © 1982 by Annual Reviews. All rights reserved

5. MOLECULAR CLOUDS IN GALACTIC DISKS

If molecular clouds are long-lived (ages 2 x 10⁸ yr), then their growth should occur as a result of collisional accretion (Kwan 1979, Scoville & Hersh 1979) and they should be present throughout the galactic disk. In this case, any observed arm-interarm contrast in CO emission would reflect primarily the extra heating that takes place near OB associations in the arm. The lack of observable arm-interarm contrast in the class 1 galaxies is consistent with the long-lived cloud hypothesis.

On the other hand, if molecular clouds are short-lived (ages several times 10⁷ yr), then they should be found in conjunction with the events that formed them. One might thus expect that the formation of molecular complexes in spiral density wave (Bash et al. 1977, Bash 1979, Elmegreen 1979, Cowie 1981) would lead to a pronounced arm-interarm contrast in CO emission. This might be avoided if the breakup of the molecular complexes as they emerge from the arm yields smaller clouds that retain their molecular character in the interarm region, preserving the same mean surface density. Observations of our Galaxy (Cohen et al. 1980) and of M31 (Stark et al. 1981) show strong contrasts in CO emission between arm (or arm segment) and interarm regions, indicating short-lived clouds. Also, the low value of _H2 / _HI in M31 indicates that the interstellar gas spends little time in the molecular phase. It may be premature, however, to extrapolate the M31 results to a global spiral pattern. The spiral arm segment mapped by Stark et al. is part of the very complex HI distribution on the southwest side of the galaxy, seen nearly tangential to the line of sight (Emerson 1978). As Unwin (1980) points out, even the atomic hydrogen does not really show a coherent spiral pattern. It might be more appropriate simply to identify the specific CO features with specific large HI structures.

It therefore appears that the degree of confinement of molecular clouds to large-scale features may depend on the molecular type of the galaxy. In an individual galaxy, the degree of confinement might further depend on galactocentric radius. Such dependences would arise if the formation rate of molecular cloud complexes in spiral arms is tied to the overall gas density (e.g. Elmegreen 1979, Cowie 1981).

CO observations of several barred galaxies (Elmegreen & Elmegreen 1982, Rickard et al., in preparation) show no close association of molecular material with the bars, although the bars generally have prominent dust lanes. Because these barred spirals are relatively distant, however, emission specifically associated with the dust lanes may be severely beam-diluted. In one giant barred spiral (NGC 1300), Elmegreen & Elmegreen made a sensitive search for CO emission in the direction of the spectacular clusters of HII regions at the ends of the bar. Their null result is surprising.

One rather clear observational result is that CO peaks are particularly associated with giant complexes of OB associations, HII regions, dust, and HI. Specific examples are seen in NGC 6946 and IC 342 (Morris & Lo 1978, Rickard & Palmer 1981b), in M101 (Blitz et al. 1981), and in M31 (Blitz, in preparation). Indeed, the curious cases are those complexes that do not have detectable CO (Rickard et al. 1977a, Blitz et al. 1981, Elmegreen & Elmegreen 1982). For example, the great HII complex 30 Dor in the LMC is only a weak source of CO emission (Israel et al. 1982). Israel et al. suggest that the molecular clouds have been broken up by interactions with the ionized gas. Blitz (private communication) suggests a similar effect in the NGC 604 complex in M33.

Blitz (in preparation) has surveyed the brighter HII regions and OB associations of M31 in order to compare their associated molecular clouds with the Giant Molecular Clouds (GMC) of our Galaxy. The intensities of the detected lines are as expected for galactic GMCs with the appropriate beam dilution, but the lines are rather wide (V ~ 15 km s^-1), which Blitz attributes to a blending of several GMCs in the beam. The distribution of GMCs in M31 resembles that of our Galaxy - an annulus peaked at ~ 9 kpc from the center.

On the other hand, M101 seems to contain at least one molecular cloud complex that is rather different from GMCs in the galactic disk. Blitz et al. (1981) find that the mass of the NGC 5461 complex is very large (10⁷-10⁸ M).