Millimeter dust emission compared with other mass estimates in N11 molecular clouds in the LMC

Abstract

CO and dust emission at millimeter wavelengths are independent tracers of cold interstellar matter, which have seldom been compared on the scale of giant molecular clouds (GMCs) in other galaxies. In this study, and for the first time for the Large Magellanic Cloud (LMC), we compute the molecular cloud masses from the millimeter emission of the dust and compare them with the masses derived from their CO luminosity and virial theorem. We present CO (J = 1–0 and J = 2–1) and 1.2 mm continuum observations of the N11 star-forming region in the LMC obtained with the SEST telescope and the SIMBA bolometer, respectively. We use the CO data to identify individual molecular clouds and measure their physical properties (CO luminosity, size, line width, and virial masses). The correlations between the properties of the N11 clouds agree with those found in earlier studies in the LMC that sample a larger set of clouds and a wider range of cloud masses. For the N11 molecular clouds, we compare the masses estimated from the CO luminosity (XCO LCO), the virial theorem (Mvir) and the millimeter dust luminosity (L1.2 mm(dust)). The measured ratios LCO/Mvir and L1.2 mm(dust)/Mvir constrain the XCO and Kdust (dust emissivity at 1.2 mm per unit gas mass) parameters as a function of the virial parameter αvir. The comparison between the different mass estimates yields a XCO-factor of 8.8 ± 3.5 × 1020 cm−2 (K km s−1) −1 α−1 vir and a Kdust parameter of 1.5 ± 0.5 × 10−3 cm2 g−1 αvir. We compare our N11 results with a similar analysis for molecular clouds in the Gould Belt in the solar neighborhood. We do not find a large discrepancy in N11 between the dust millimeter and virial masses as reported in earlier studies of molecular clouds in the Small Magellanic Cloud. The ratio between L1.2 mm and Mvir in N11 is half of what is measured for Gould Belt clouds, which can be accounted for by a factor of two lower gas-to-dust mass ratio, as the difference in gas metallicities. If the two samples have similar αvir values, this result implies that their dust far-IR properties are also similar.