![]() | Annu. Rev. Astron. Astrophys. 1982. 20:
517-45 Copyright © 1982 by Annual Reviews. All rights reserved |
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).