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Description
What are the mechanisms by which massive stars form? What are the initial conditions for these processes? It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs) likely represent the birth sites massive stars. Therefore, this class of objects gets increasing attention, and their analysis offers the opportunity to tackle the above mentioned questions. To enlarge the sample of well-characterised IRDCs in the southern hemisphere, where ALMA will play a major role in the near future, we have set up a program to study the gas and dust of southern infrared dark clouds. The present paper aims at characterizing the continuum properties of this sample of IRDCs. We cross-correlated 1.2 mm continuum data from SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between emission sources at millimeter wavelengths a nd the IRDCs we see at 8{mu}m in absorption against the bright PAH background. Analysing the dust emission and extinction leads to a determination of masses and column densities, which are important quantities in characterizing the initial conditions of massive star formation. We also evaluated the limitations of the emission and extinction methods. The morphology of the 1.2mm continuum emission is in all cases in close agreement with the mid-infrared extinction. The total masses of the IRDCs were found to range from 150 to 1150M_{sun}_ (emission data) and from 300 to 1750M_{sun}_ (extinction data). We derived peak column densities between 0.9 and 4.6x10^22^cm^-2^ (emission data) and 2.1 and 5.4x10^22^cm^-2^ (extinction data). We demonstrate that the extinction method fails for very high extinction values (and column densities) beyond A_V_ values of roughly 75mag according to the Weingartner & Draine (2001ApJ...548..296W) extinction relation R_V_=5.5 model B (around 200mag when following the common Mathis (1990ARA&A..28...37M) extinction calibration). By taking the spatial resolution effects into account and restoring the column densities derived from the dust emission back to a linear resolution of 0.01pc, peak column densities of 3.0x10^23^cm^-2^ are obtained, much higher than typic al values for low-mass cores. The derived column densities, taking into account the spatial resolution effects, are beyond the column density threshold of 3.0x10^23^cm^-2^ required by theoretical considerations for massive star formation. We conclude that the values for column densities derived for the selected IRDC sample make these objects excellent candidates for objects in the earliest stages of massive star formation.
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