Physical conditions of the molecular gas in metal-poor galaxies
  Studying the molecular component of the interstellar medium in metal-poor
galaxies has been challenging because of the faintness of carbon monoxide
emission, the most common proxy of H2. Here we present new detections of
molecular gas at low metallicities, and assess the physical conditions in the
gas through various CO transitions for 8 galaxies. For one, NGC 1140 (Z/Zsun ~
0.3), two detections of 13CO isotopologues and atomic carbon, [CI](1-0), and an
upper limit for HCN(1-0) are also reported. After correcting to a common beam
size, we compared 12CO(2-1)/12CO(1-0) (R21) and 12CO(3-2)/12CO(1-0) (R31) line
ratios of our sample with galaxies from the literature and find that only NGC
1140 shows extreme values (R21 ~ R31 ~ 2). Fitting physical models to the 12CO
and 13CO emission in NGC 1140 suggests that the molecular gas is cool (kinetic
temperature Tkin<=20 K), dense (H2 volume density nH2 >= $10^6$ cm$^{-3}$),
with moderate CO column density (NCO ~ $10^{16}$ cm$^{-2}$) and low filling
factor. Surprisingly, the [12CO]/[13CO] abundance ratio in NGC 1140 is very low
(~ 8-20), lower even than the value of 24 found in the Galactic Center. The
young age of the starburst in NGC 1140 precludes 13C enrichment from evolved
intermediate-mass stars; instead we attribute the low ratio to charge-exchange
reactions and fractionation, because of the enhanced efficiency of these
processes in cool gas at moderate column densities. Fitting physical models to
12CO and [CI](1-0) emission in NGC 1140 gives an unusually low [12CO]/[12C]
abundance ratio, suggesting that in this galaxy atomic carbon is at least 10
times more abundant than 12CO.
