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 in 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 are in agreement with those found in earlier studies in the LMC that sample a larger set of clouds and a larger 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 ( \mathcal { L } _ { 1.2 mm } ( { dust } ) ) .
The measured ratios L _ { CO } / M _ { vir } and \mathcal { L } _ { 1.2 mm } ( { dust } ) / M _ { vir } constrain the X _ { CO } and \mathcal { K } _ { dust } ( dust emissivity at 1.2 mm per unit gas mass ) parameters as a function of the virial parameter \alpha _ { vir } .
The comparison between the different mass estimates yields a X _ { CO } -factor of 8.8 \pm 3.5 \times 10 ^ { { 20 } } cm ^ { -2 } ( K km s ^ { -1 } ) ^ { -1 } \alpha _ { vir } ^ { -1 } and a \mathcal { K } _ { dust } parameter of 1.5 \pm 0.5 \times 10 ^ { -3 } ~ { } { cm ^ { 2 } g ^ { -1 } } \alpha _ { vir } .
We compare our N11 results with a similar analysis for molecular clouds in the Gould ’ s Belt in the solar neighborhood .
We do not find in N11 a large discrepancy between the dust millimeter and virial masses as reported in earlier studies of molecular clouds in the Small Magellanic Cloud .
The ratio between \mathcal { L } _ { 1.2 mm } and M _ { vir } in N11 is half of that measured for Gould ’ s 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 \alpha _ { vir } values , this result implies that their dust far-IR properties are also similar .