Characterizing the Transition from Diffuse Atomic to Dense Molecular Clouds in the Magellanic Clouds with [C ii], [C i], and CO

Abstract

We present and analyze deep Herschel/HIFI observations of the [C ii] 158 µm, [C i] 609 µm, and [C i] 370 µm lines towards 54 lines-of-sight (LOS) in the Large and Small Magellanic clouds. 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 C 0 to CO in their low metallicity environments. We trace gas with molecular fractions in the range 0.1 < f (H 2 ) < 1, between those in the diffuse H 2 gas detected by UV absorption (f (H 2 ) < 0.2) and well shielded regions in which hydrogen is essentially completely molecular. The C 0 and CO column densities are only measurable in regions with molecular fractions f (H 2 ) > 0.45 in both the LMC and SMC. Ionized carbon is the dominant gas-phase form of this element that is associated with molecular gas, with C 0 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 H 2 . 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 LOSs observed in both Magellanic Clouds.

Physical conditions in the gas phases of the giant H II region LMC-N 11

“…A somewhat warmer temperature is expected in metal-poor environments due to the larger photoelectric-effect heating efficiency (itself due to the low dust content), to the lack of metal coolants, and to the harder interstellar radiation field (ISRF). We assume in the following a temperature of 50 K in the molecular phase but our results are not changed significantly if we take 100 K. At thermal pressure equilibrium, the density in the H 2 gas is therefore twice larger than in the H 0 gas, reaching up to ≈ 10 3 cm −3 , on the low end of values found in Pineda et al (2017) for a sam-ple of other LMC star-forming regions. We allow the molecular gas to be denser if no solution can be found with a thermal pressure equilibrium hypothesis.…”

“…In the neutral gas, C + may exist in the H 0 phase but also in the H 2 phase, in regions where CO is photodissociated while H 2 is self-shielded and shielded by dust, the so-called CO-dark molecular gas (e.g., Madden et al 1997;Grenier et al 2005;Wolfire et al 2010). The [C ii] line has been used to trace the star-formation rate (SFR; e.g., De Looze et al 2014;Pineda et al 2014), to infer physical conditions in photodissociation regions (PDRs), and to calculate the CO-to-H 2 conversion factor (X CO ; e.g., Jameson et al 2018;Herrera-Camus et al 2017;Pineda et al 2017). The growing number of observations in both Galactic and extragalac-tic environments (with routine detections at z > 6; e.g., Aravena et al 2016) has renewed the interest in understanding [C ii] as a diagnostic tool.…”

“…Requena-Torres et al (2016) observed several star-forming regions in the Small Magellanic Cloud (SMC; ≈ 1/5 Z ) and found that most of the H 2 gas is traced by [C ii] and not CO. Using 54 lines of sight in the LMC and SMC, Pineda et al (2017) find that most of the molecular gas is COdark. Overall, the finding that CO-dark H 2 gas is predominant in low-metallicity environments is consistent with the picture of CO-emitting regions occupying a smaller filling factor due to the relatively lower dust-to-gas mass ratio, and it is in fact possible to obtain X CO conversion factors close to the Milky Way value in the Magellanic Clouds if filling factor effects are accounted for, as shown by Pineda et al (2017).…”

“…Using 54 lines of sight in the LMC and SMC, Pineda et al (2017) find that most of the molecular gas is COdark. Overall, the finding that CO-dark H 2 gas is predominant in low-metallicity environments is consistent with the picture of CO-emitting regions occupying a smaller filling factor due to the relatively lower dust-to-gas mass ratio, and it is in fact possible to obtain X CO conversion factors close to the Milky Way value in the Magellanic Clouds if filling factor effects are accounted for, as shown by Pineda et al (2017). However, the dependence of f dark gas with metallicity may be indirect, and model results from the Herschel Dwarf Galaxy Survey (DGS; Madden et al 2013) indicate that f dark is mostly a function of the effective cloud extinction, with the latter showing some dependence with metallicity (Madden et al in prep.…”

First Results from the Herschel and ALMA Spectroscopic Surveys of the SMC: The Relationship between [C ii]-bright Gas and CO-bright Gas at Low Metallicity*

“…Observationally, existing data suggest that the [C I] traces a small fraction of the total molecular gas in the SMC (Requena-Torres et al 2016; Pineda et al 2017). The theoretical work by Nordon & Sternberg (2016) and simulations by Glover et al (2015) also suggest that most of the neutral carbon will be mixed with the CO, and the amount of molecular gas traced only by [C I] is always much less than that traced by [C II] or CO.…”