Abstract
We present a study of the infrared/submm emission of the LMC star forming complex N158-N159-N160. Combining observations from the Spitzer Space Telescope (3.6-70um), the Herschel Space Observatory (100-500um) and LABOCA (870um) allows us to work at the best angular resolution available now for an extragalactic source. We observe a remarkably good correlation between 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 MBB, we derive a global emissivity index beta_c of 1.47. If beta cold is fixed to 1.5, we find an average temperature of 27K. We also apply the Galliano et al. (2011) modelling technique (and amorphous carbon to model carbon dust) to derive maps of the star formation rate, the mean starlight intensity, the fraction of PAHs or the dust mass surface density of the region. We observe that the PAH fraction strongly decreases in the HII regions. This decrease coincides with peaks in the mean radiation field intensity map. The dust surface densities follow the FIR distribution, with a total dust mass of 2.1x10^4 Msolar (2.8 times less than when using graphite grains) in the resolved elements we model. We find a non-negligible amount of dust in the molecular cloud N159 South (showing no massive SF). We also investigate the drivers of the Herschel/PACS and SPIRE submm colours as well as the variations in the gas-to-dust mass ratio (G/D) and the XCO conversion factor in the region N159. We finally model individual regions to analyse variations in the SED shape across the complex and the 870um emission in more details. No measurable submm excess emission at 870um seems to be detected in these regions.
Highlights
As potential templates of primordial environments, low-metallicity galaxies are keystones to understand how galaxies evolve through cosmic time
N160 shows the highest cold dust temperature, with a maximum of ∼40K while the median temperature of the N158-N159-N160 complex is 26.9±2.3K (28.2 if we restrict the analysis to interstellar medium (ISM) elements with a 3-σ detection in the SPIRE bands)
In order to investigate the drivers of submm colours variations in the N158-N159-N160 complex, we compare in Fig. 5 the PACS and SPIRE surface brightness ratios 100/160, 160/250, 250/350 and 350/500 with : 1) the 3.6 μm surface brightness: dominated by emission from the old stellar populations, it is known as a good proxy of the stellar mass, 2) the 24 μm hot dust emission, often used as a good calibrator for star formation (Calzetti 2007; Rieke et al 2009, among others), 3) the mean starlight heating intensity
Summary
As potential templates of primordial environments, low-metallicity galaxies are keystones to understand how galaxies evolve through cosmic time. More relevant to the present study, the SEDs of low-metallicity galaxies often exhibit a flattening of their submillimeter (submm) slope or a submm excess (Bottner et al 2003; Galliano et al 2003, 2005; Marleau et al 2006; Bendo et al 2006; Galametz et al 2009, 2011), namely a higher emission beyond 500 μm than that extrapolated from IR observations and standard dust properties (Milky Way dust for instance) The origin of this excess is still highly debated. This paper presents a study of the IR to submm thermal emission of star-forming regions in various evolutionary states at the best resolution available This allows us to investigate how properties and physical conditions observed on large scales are physically linked to specific environments on a more resolved scale.
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