2017ApJ...839..107P

We present and analyze deep Herschel/HIFI observations of the [C II] 158 µm, [C i] 609 µm, and [C i] 370 µm lines toward 54 lines of sight in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). These observations are used to determine the physical conditions of the line-emitting gas, which we use to study the transition from atomic to molecular gas and from C⁺ to C⁰ to CO in their low-metallicity environments. We trace gas with molecular fractions in the range 0.1 f(H₂) 1, between those in the diffuse H₂ gas detected by UV absorption (f(H₂) 0.2) and well-shielded regions in which hydrogen is essentially completely molecular. The C⁰ and CO column densities are only measurable in regions with molecular fractions f(H₂) 0.45 in both the LMC and the SMC. Ionized carbon is the dominant gas-phase form of this element that is associated with molecular gas, with C⁰ and CO representing a small fraction, implying that most (89% in the LMC and 77% in the SMC) of the molecular gas in our sample is CO-dark H₂. The mean X_CO conversion factors in our LMC and SMC sample are larger than the value typically found in the Milky Way. When applying a correction based on the filling factor of the CO emission, we find that the values of X_CO in the LMC and SMC are closer to that in the Milky Way. The observed [C II] intensity in our sample represents about 1% of the total far-infrared intensity from the lines of sight observed in both Magellanic clouds.