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

¹ Institut d’Astrophysique Spatiale (IAS), UMR 8617, Université Paris-Sud 11, Bâtiment 121, 91405 Orsay Cedex, France
e-mail: cherrera@ias.u-psud.fr
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Las Condes, Santiago, Chile
³ Department of Astronomy and Laboratory for Millimeter-Wave Astronomy, University of Maryland, College Park, MD 20742, USA
⁴ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena, Chile

Received: 10 April 2012
Accepted: 28 March 2013

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 (X_CO   L_CO), the virial theorem (M_vir) and the millimeter dust luminosity (ℒ_1.2   mm(dust)). The measured ratios L_CO/M_vir and ℒ_1.2   mm(dust)/M_vir constrain the X_CO and (dust emissivity at 1.2 mm per unit gas mass) parameters as a function of the virial parameter α_vir. The comparison between the different mass estimates yields a X_CO-factor of 8.8 ± 3.5    × 10²⁰ cm^-2 (K km s^-1) ^-1 α_vir^-1 and a parameter of 1.5 ± 0.5    × 10^-3   cm²   g^-1   α_vir. 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 ℒ_1.2   mm and M_vir 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 α_vir values, this result implies that their dust far-IR properties are also similar.