2017A&A...606A..99H

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 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/Z_☉∼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 T_kin≥20K), dense (H₂ volume density n_H2≥10⁶cm^–3), with moderate CO column density (N_CO∼10¹⁶cm^–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 ¹³CO 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.