Abstract
Studying the molecular component of the interstellar medium (ISM) in metal-poor galaxies has been challenging because of the faintness of carbon monoxide emission, the most common proxy of H-2. 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/Z(circle dot) similar to 0.3), two detections of (CO)-C-13 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 (CO)-C-12(2-1)/(CO)-C-12(1-0) (R (21)) and (CO)-C-12(3-2)/(CO)-C-12(1-0) (R-31) line ratios of our sample with galaxies from the literature and find that only NGC 1140 shows extreme values (R-21 similar to R-31 similar to 2). Fitting physical models to the (CO)-C-12 and (CO)-C-13 emission in NGC 1140 suggests that the molecular gas is cool (kinetic temperature T-kin less than or similar to 20 K), dense (H-2 volume density n(H2) greater than or similar to 10(6) cm (3)), with moderate CO column density (N-CO similar to 10(16) cm(-2)) and low filling factor. Surprisingly, the [(CO)-C-12]/[(CO)-C-13] abundance ratio in NGC 1140 is very low (similar to 8-20), lower even than the value of 24 found in the Galactic Center. The young age of the starburst in NGC 1140 precludes (CO)-C-13 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 (CO)-C-12 and [CI](1-0) emission in NGC 1140 gives an unusually low [(CO)-C-12]/[C-12] abundance ratio, suggesting that in this galaxy atomic carbon is at least 10 times more abundant than (CO)-C-12.