Abstract
With an aim of probing the physical conditions and excitation mechanisms of warm molecular gas in individual star-forming regions, we performed Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of 30 Doradus in the Large Magellanic Cloud. In our FTS observations, important far-infrared (FIR) cooling lines in the interstellar medium, including CO J = 4–3 to J = 13–12, [C I] 370 μm, and [N II] 205 μm, were clearly detected. In combination with ground-based CO J = 1–0 and J = 3–2 data, we then constructed CO spectral line energy distributions (SLEDs) on ~10 pc scales over a ~60 pc × 60 pc area and found that the shape of the observed CO SLEDs considerably changes across 30 Doradus. For example, the peak transition Jp varies from J = 6–5 to J = 10–9, while the slope characterized by the high-to-intermediate J ratio α ranges from ~0.4 to ~1.8. To examine the source(s) of these variations in CO transitions, we analyzed the CO observations, along with [C II] 158 μm, [C I] 370 μm, [O I] 145 μm, H2 0–0 S(3), and FIR luminosity data, using state-of-the-art models of photodissociation regions and shocks. Our detailed modeling showed that the observed CO emission likely originates from highly compressed (thermal pressure P∕kB ~ 107–109 K cm−3) clumps on ~0.7–2 pc scales, which could be produced by either ultraviolet (UV) photons (UV radiation field GUV ~ 103–105 Mathis fields) or low-velocity C-type shocks (pre-shock medium density npre ~ 104–106 cm−3 and shock velocity vs ~ 5–10 km s−1). Considering the stellar content in 30 Doradus, however, we tentatively excluded the stellar origin of CO excitation and concluded that low-velocity shocks driven by kiloparsec-scale processes (e.g., interaction between the Milky Way and the Magellanic Clouds) are likely the dominant source of heating for CO. The shocked CO-bright medium was then found to be warm (temperature T ~ 100–500 K) and surrounded by a UV-regulated low-pressure component (P∕kB ~ a few (104 –105) K cm−3) that is bright in [C II] 158 μm, [C I] 370 μm, [O I] 145 μm, and FIR dust continuum emission.
Highlights
As a nascent fuel for star formation, molecular gas plays an important role in the evolution of galaxies (e.g., Kennicutt &Evans 2012)
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. 1 In this paper, we focus on 12CO and refer to it as CO
The authors performed the observations as a single pointing toward (α, δ)J2000 = (05h38m48.6s, −69◦04 43.2 ), and we found that the NANTEN2-to-Fourier Transform Spectrometer (FTS) ratios of the integrated intensities for this position are 1.2, suggesting that our intensity measurements are consistent with Pineda et al (2012) within 1σ uncertainties
Summary
As a nascent fuel for star formation, molecular gas plays an important role in the evolution of galaxies The original PACS images were smoothed and rebinned to match the FTS resolution (42 ) and pixel size (30 ) This smoothing and rebinning procedure resulted in a total of 13 common pixels to work with (e.g., Fig. 7; mainly limited by the small coverage of the [O I] 145 μm data), over which [C II] 158 μm and [O I] 145 μm were clearly detected with S /Ns 5. The CO(3–2) data were obtained by Minamidani et al (2008) on 22 scales using the 10 m Atacama Submillimeter Telescope Experiment (ASTE) telescope For both datasets, the final uncertainties in the integrated intensities were estimated in a similar manner to that used for our FTS CO observations: adding the statistical 5 http://irs.sirtf.com/IRS/SmartRelease. For details on our dust SED modeling, we refer to Galliano (2018)
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