Abstract
We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In our observations, a number of far-infrared cooling lines including CO(4-3) to CO(12-11), [CI] 609 and 370 micron, and [NII] 205 micron are clearly detected. With an aim of investigating the physical conditions and excitation processes of molecular gas, we first construct CO spectral line energy distributions (SLEDs) on 10 pc scales by combining the FTS CO transitions with ground-based low-J CO data and analyze the observed CO SLEDs using non-LTE radiative transfer models. We find that the CO-traced molecular gas in N159W is warm (kinetic temperature of 153-754 K) and moderately dense (H2 number density of (1.1-4.5)e3 cm-3). To assess the impact of the energetic processes in the interstellar medium on the physical conditions of the CO-emitting gas, we then compare the observed CO line intensities with the models of photodissociation regions (PDRs) and shocks. We first constrain the properties of PDRs by modelling Herschel observations of [OI] 145, [CII] 158, and [CI] 370 micron fine-structure lines and find that the constrained PDR components emit very weak CO emission. X-rays and cosmic-rays are also found to provide a negligible contribution to the CO emission, essentially ruling out ionizing sources (ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source for CO in N159W. On the other hand, mechanical heating by low-velocity C-type shocks with ~10 km/s appears sufficient enough to reproduce the observed warm CO.
Highlights
Star formation exclusively occurs in molecular clouds, the densest component of the interstellar medium (ISM; e.g., Kennicutt & Evans 2012)
We find that the best-fit parameters for the models with GX = 0 and GX = 10 are comparable, implying that X-rays from the nearby Large Magellanic Cloud (LMC) X-1 do not make a significant impact on the photodissociation regions (PDRs) tracers used in our analysis
For the carbon monoxide (CO)-bright layers (AV ∼ 2 mag), the temperatures reach up to ∼180 K and ∼800 K for vs = 6 km s−1 and 14 km s−1 respectively and the postshock densities are intermediate with a few × 104 cm−3, which are in reasonably good agreement with our RADEX-based estimates (Sect. 4.2)
Summary
Star formation exclusively occurs in molecular clouds, the densest component of the interstellar medium (ISM; e.g., Kennicutt & Evans 2012). We study N159W, an active star-forming region in the Large Magellanic Cloud (LMC), largely based on Herschel PACS and SPIRE observations. N159W is one of the three prominent molecular clouds in the HII region complex N159 (Fig. 1) and its stellar and gas contents have been extensively studied at multiple wavelengths. De Boer et al (1998) proposed that the motion of the LMC through the hot halo gas of the Milky Way created bow shocks at the leading edge, triggering the sequential star formation This leading edge of the LMC corresponds to the southeastern HI overdensity region, which appears to merge into the Small Magellanic Cloud (SMC) through the Magellanic Bridge connecting the two Magellanic Clouds (e.g., Kim et al 1998; Putman et al 2003). We describe the data in our study and summarize their main parameters (e.g., rest wavelength, FWHM, 1σ uncertainty in the integrated intensity, luminosity, etc.; Table 1)
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