  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. To enlarge the sample of well-characterised
IRDCs in the southern hemisphere, we have set up a program to study the gas and
dust of southern IRDCs. The present paper aims at characterizing the continuuum
properties of this sample of objects. 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 and 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. The total masses of the IRDCs were found to range from 150 to 1150
$\rm M_\odot$ (emission data) and from 300 to 1750 $\rm M_\odot$ (extinction
data). We derived peak column densities between 0.9 and 4.6 $\times 10^{22}$
cm$^{-2}$ (emission data) and 2.1 and 5.4 $\times 10^{22}$ cm$^{-2}$
(extinction data). We demonstrate that the extinction method fails for very
high extinction values (and column densities) beyond A$_{\rm V}$ values of
roughly 75 mag according to the Weingartner & Draine (2001) extinction relation
$R_{\rm V} = 5.5$ model B. The derived column densities, taking into account
the spatial resolution effects, are beyond the column density threshold of 3.0
$\times 10^{23}$ 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.

Solar and Stellar Astrophysics