The rings are gas rich but seemingly H2 poor. Since HI
rapidly converts into H2,
the low fmol cannot signal the consumption of the
molecular gas reservoir. Nor
can metallicity effects by themselves be responsible, at least in
L-S. The gas phase
pressure (PISM) might be a factor, as it directly
affects the HI to H2 conversion rate.
Elmegreen (1993)
finds fmol
(PISM /
P
)2.2
(
/
)-1,
where
is the ambient
UV-field. For L-S's ring we
estimated
with Spitzer and GALEX images, and PISM
(
G/2)(
gas2
+ (
gas /
*)
gas
*),
with the ring's
stellar surface mass density derived with IRAC 4.5 µm
data. We find very high gas phase pressures, with PISM /
PISM,local
30-400, leading us
to expect fmol
1
everywhere. L-S's ring should be dominated by molecular
gas. Why isn't it?
We used photo-dissociation models in
Allen et al. (2004)
to estimate average gas volume densities (n) in L-S's ring given its
HI and
UV-field. In the northern half of the ring,
n = 100-300 cm-3, implying an ISM dominated by the
Cold Neutral
Medium (CNM, T = 50-100 K), i.e., the precursor of
cold molecular clouds. In the southwest,
where
SFR
and
HI
are both much higher, the models give
n
2 cm-3, which taken at face value, points to an ISM dominated
by the Warm Neutral Medium (WNM, T
7000 K). How can you
form stars out of this?
We believe the answer lies in fundamental differences in the
environments of rings and spiral arms. Consider that molecular clouds spend
20 Myrs in the arms
of grand design spirals like M 51, whereas the ISM is confined in rings,
equally as dense and actively forming stars, for
200 Myr. While
molecular cloud growth is enhanced in the high
gas
rings, the destructive effects of SNe
and OB stars are also amplified. A dominant WNM might be
expected as the molecular clouds
become fragmented and "over-cooked" by shocks and sustained UV-fields.
CO might in this case retreat to the inner-most cloud cores resulting in
weak ICO and underestimates of
H2. This might
explain the peculiar Schmidt Laws and enhanced SFE. At the same
time, higher cloud collision rates might favor the formation of
unusually large molecular cloud complexes and more efficient star
formation. More work remains though results from Cartwheel and
L-S are intriguing.