The thermal dust emission in N158–N159–N160 (LMC) star-forming complex mapped by Spitzer, Herschel and LABOCA

Abstract

Low-metallicity galaxies exhibit different properties of the interstellar medium (ISM) compared to nearby spiral galaxies. Obtaining a resolved inventory of the various gas and dust components of massive star-forming regions and diffuse ISM is necessary to understand how those differences are driven. We present a study of the infrared/submillimetre (submm) emission of the massive star-forming complex N158–N159–N160 located in the Large Magellanic Cloud (LMC). Combining observations from the Spitzer Space Telescope (3.6–70 µm), the Herschel Space Observatory (100–500 µm) and Large APEX Bolometer Camera (LABOCA) (on Atacama Pathfinder EXperiment, 870 µm) allows us to work at the best angular resolution available now for an extragalactic source (a few parsec for the LMC). We observe a remarkably good correlation between the Herschel Spectral and Photometric Imaging Receiver (SPIRE) and LABOCA emission and resolve the low surface brightnesses emission. We use the Spitzer and Herschel data to perform a resolved spectral energy distribution (SED) modelling of the complex. Using modified blackbodies, we derive an average ‘effective’ emissivity index of the cold dust component βc of 1.47 across the complex. If βc is fixed to 1.5, we find an average temperature of ∼27 K (maximum of ∼32 K in N160). We also apply the Galliano et al. SED modelling technique (using amorphous carbon to model carbon dust) to derive maps of the star formation rate, the grain temperature, the mean starlight intensity, the fraction of polycyclic aromatic hydrocarbons (PAH) or the dust mass surface density of the region. We observe that the PAH fraction strongly decreases in the H II regions we study. This decrease coincides with peaks in the mean radiation field intensity map. The dust surface densities follow the far-infrared distribution, with a total dust mass of 2.1 × 104 M (2.8 times less than if carbon dust was modelled by standard graphite grains) in the resolved elements we model. We also find a non-negligible amount of dust in the region called ‘N159 South’, a molecular cloud that does not show massive star formation. We also investigate the drivers of the Herschel/PACS (Photodetector Array Camera and Spectrometer) and SPIRE submm colours and find that the submm ratios correlate strongly with the radiation field intensity and with the near and mid-IR surface brightnesses equally well. Comparing our dust map to H I and CO observations in N159, we then investigate variations in the gas-to-dust mass ratio (G/D) and the CO-to-H2 conversion factor XCO. A mean value of G/D∼356 is derived when using XCO = 7×1020 H2 cm−2 (K km s−1) −1. If a constant G/D across N159 is assumed, we derive a XCO conversion factor of 5.4×1020 H2 cm−2 (K km s−1) −1. We finally model individual regions to analyse variations in the SED shape across the complex and the 870 µm emission in more details. No measurable submm excess emission at 870 µm seems to be detected in these regions.