Velocity profiles of [C_II], [C_I], CO, and [O_I] and physical conditions in four star-forming regions in the Large Magellanic Cloud

Abstract

Aims. The aim of our study is to investigate the physical properties of the star-forming interstellar medium (ISM) in the Large Magellanic Cloud (LMC) by separating the origin of the emission lines spatially and spectrally. The LMC provides a unique local template to bridge studies in the Galaxy and high redshift galaxies because of its low metallicity and proximity, enabling us to study the detailed physics of the ISM in spatially resolved individual star-forming regions. Following Okada et al. (2015, Paper I), we investigate different phases of the ISM traced by carbon-bearing species in four star-forming regions in the LMC, and model the physical properties using the KOSMA-τ PDR model. Methods. We mapped 3-13 arcmin 2 areas in 30 Dor, N158, N160 and N159 along the molecular ridge of the LMC in [C ii] 158 µm with GREAT on board SOFIA. We also observed the same area with CO(2-1) to (6-5), 13 CO(2-1) and (3-2), [C i] 3 P 1 -3 P 0 and 3 P 2 -3 P 1 with APEX. For selected positions in N159 and 30 Dor, we observed [O i] 145 µm and [O i] 63 µm with upGREAT. All spectra are velocity resolved. Results. In all four star-forming regions, the line profiles of CO, 13 CO, and [C i] emission are similar, being reproduced by a combination of Gaussian profiles defined by CO(3-2), whereas [C ii] typically shows wider line profiles or an additional velocity component. At several positions in N159 and 30 Dor, we observed the velocity-resolved [O i] 145 µm and 63 µm lines for the first time. At some positions, the [O i] line profiles match those of CO, at other positions they are more similar to the [C ii] profiles. We interpret the different line profiles of CO, [C ii] and [O i] as contributions from spatially separated clouds and/or clouds in different physical phases, which give different line ratios depending on their physical properties. We model the emission from the CO, [C i], [C ii], and [O i] lines and the far-infrared continuum emission using the latest KOSMA-τ PDR model, which treats the dust-related physics consistently and computes the dust continuum SED together with the line emission of the chemical species. We find that the line and continuum emissions are not well-reproduced by a single clump ensemble. Toward the CO peak at N159 W, we propose a scenario that the CO, [C ii], and [O i] 63 µm emission are weaker than expected because of mutual shielding among clumps.

The CO-dark molecular gas mass in 30 Doradus

“…However, the CO lines are better reproduced when using an isobaric model, which is what is driving our choice for an isobaric model here. Okada et al (2019) find G UV between ∼ 10 2 and 10 5.5 and densities n between ∼ 10 3.5 and 10 7 cm −3 throughout 30Dor, with similar distributions as in Figure 4. The values of n (Okada et al 2019) and P (this study) are not straightforward to compare, but we note that the values of G UV derived with the KOSMA-τ PDR model reach much higher values than that constrained with the Meudon PDR model, which seem inconsistent with the emitted stellar radiation derived in Chevance et al (2016).…”

“…The values of n (Okada et al 2019) and P (this study) are not straightforward to compare, but we note that the values of G UV derived with the KOSMA-τ PDR model reach much higher values than that constrained with the Meudon PDR model, which seem inconsistent with the emitted stellar radiation derived in Chevance et al (2016). These could result from the degeneracy between a high n-high G UV and a low n-low G UV solution as noted by Okada et al (2019).…”

“…Recent observations of several regions of the LMC, including (Risacher et al 2016). These observations are presented by Okada et al (2019), who combined them with the CO transitions from J = 2-1 to J = 6-5, [C I] 3 P 1 − 3 P 0 , and [C I] 3 P 2 − 3 P 1 , to constrain the parameters of the KOSMA-τ PDR model (Störzer et al 1996;Röllig et al 2006;Röllig et al 2013), at 30 resolution. We note that they use an isochoric PDR model (with a constant gas density through a given PDR cloud), while we use an isobaric model (with a constant pressure through a given PDR cloud).…”

Molecular gas properties of Q1700-MD94: A massive main-sequence galaxy at z ≈ 2

“…In contrast to what is expected from the classical onion-like structure of photodissociation regions (PDRs; e.g., Hollenbach & Tielens 1999), more recent simulations suggest that neutral carbon emission appears to be widespread in H 2 clouds (e.g., Offner et al 2014;Glover & Clark 2016;Bisbas et al 2017). Observations that support this scenario include large-scale maps of the CO isotopologs and [CI] lines of star-forming regions in the Small and Large Magellanic Clouds (Requena-Torres et al 2016;Okada et al 2019) and in the ρ Ophiuchi (Kulesa et al 2005) and Orion giant molecular clouds. For the latter, the [CI](1-0) emission is measured to be optically thin (τ [CI] ∼ 0.1 − 0.3), and coincident with the emission of the 12 CO line and 13 CO(1-0) lines (Ikeda et al 2002;Shimajiri et al 2013).…”

Molecular gas properties of Q1700-MD94: a massive, main-sequence galaxy at $z\approx2$

“…In contrast to what is expected from the classical onion-like structure of photodissociation regions (PDRs; e.g., Hollenbach & Tielens 1999), more recent simulations suggest that neutral carbon emission appears to be widespread in H 2 clouds (e.g., Offner et al 2014;Glover & Clark 2016;Bisbas et al 2017). Observations that support this scenario include large scale maps of the CO isotopologues and [CI] lines of star-forming regions in the Small and Large Magellanic Clouds (Requena-Torres et al 2016;Okada et al 2019), and the ρ Ophiuchi (Kulesa et al 2005) and Orion giant molecular clouds. For the latter, the [CI](1-0) emission is measured to be optically thin (τ [CI] ∼ 0.1 − 0.3), and coincident with the emission of the 12 CO line and 13 CO(1-0) lines (Ikeda et al 2002;Shimajiri et al 2013).…”