Physical properties of Southern infrared dark clouds

Context. 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) represent the birth-sites of massive stars. Therefore, these clouds have been receiving an increasing amount of attention, and their analysis offers the opportunity to tackle the afore mentioned questions. Aims. 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 developed a program to study the gas and dust of southern infrared dark clouds. The present paper attempts to characterize the continuum properties of this sample of IRDCs. Methods. We cross-correlated 1.2 mm continuum data from SIMBA bolometer array mounted on SEST telescope with Spitzer/GLIMPSE images to establish the connection between emission sources at millimeter wavelengths and the IRDCs that we observe at 8 mu m in absorption against the bright PAH background. Analysing the dust emission and extinction enables us to determine the 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. Results. The morphology of the 1.2 mm 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 1150 M-circle dot (emission data) and from 300 to 1750 M-circle dot (extinction data). We derived peak column densities of between 0.9 and 4.6 x 10(22) cm(-2) (emission data) and 2.1 and 5.4 x 10(22) cm(-2) (extinction data). We demonstrate that the extinction method is unreliable at very high extinction values (and column densities) beyond A(V) values of roughly 75 mag according to the Weingartner & Draine (2001) extinction relation R-V = 5.5 model B (around 200 mag when following the common Mathis (1990, ApJ, 548, 296) extinction calibration). By taking the spatial resolution effects into account and restoring the column densities derived from the dust emission to a linear resolution of 0.01 pc, peak column densities of 3-19 x 10(23) cm(-2) are obtained, which are much higher than typical values for low-mass cores. Conclusions. Taking into account the spatial resolution effects, the derived column densities are beyond the column density threshold of 3.0 x 10(23) cm(-2) required by theoretical considerations for massive star formation. We conclude that the values of column densities derived for the selected IRDC sample imply that these objects are excellent candidates for objects in the earliest stages of massive star formation.