What evidence do we have that significant quantities of molecular gas
could be ``hiding'' in the ISM of many galaxies? Well, we do know that
many galaxies contain dark matter, and at present there seems to be
nothing wrong with assuming that at least some part of that
dark matter is in the form of a disk of molecular gas. Low-density
molecular gas could escape detection since the excitation of the CO
molecules may be subthermal, but in that case it is not likely that
much mass is involved. If gravitationally-significant amounts of
H2 are present, say M(H2)
M(HI),
then the molecular gas would
have to be more dense, and also generally cold in order to escape
detection in the extensive CO(1-0) emission line surveys of galaxy
disks. Such gas could exist in well-shielded parts of galaxies where
the UV flux and cosmic ray density are both very low.
The inner disk of M31 is just such a ``cold'' environment, and it was here where an initial detection of faint CO emission emanating from large dark dust clouds was first reported (Allen & Lequeux 1993). Modelling of this data (Allen et al. 1995) confirmed that the ``cold gas'' interpretation was viable. Subsequent observations discovered similar emission in other dust clouds in the inner disk (Loinard, Allen, & Lequeux 1996), and also showed that such emission is ubiquitous in the inner disk of M31 (Loinard, Allen, & Lequeux 1995), covering more than half of the annular area between about 1 and 8 kpc in galactocentric radius. The distinction between cold, extensive gas and small, warm, beam-diluted condensations was made in favor of the former with a combination of JCMT CO(3-2) and OVRO CO(1-0) synthesis-imaging observations on the dust cloud D478 by Loinard & Allen (1998). The bulk of the molecular gas in this GMC-sized cloud appears as an absorbing screen at ~ 3.5 K. There is no reason to presume that D478 is peculiar, so we must conclude that cold molecular gas is indeed extensively present in the inner disk of M31.
How much mass could be in such a component?
Loinard & Allen
(1998)
suggested that if the GMCs were at least marginally bound, then
H2 mass surface densities of order ~ 100
M
pc-2 would
be present in the inner disk of M31. However, very recently
Pringle, Allen, &
Lubow (2000)
have argued that GMCs are unbound, transient
objects that are not formed from in situ cooling of
HI, but
from the agglomeration of the dense phase of the ISM, much of which is
already molecular. How much mass is in this component is presently the
big question, but it's a delicate one that needs some careful thought,
and I am not prepared to address it here any further just yet.