Mass reservoirs surrounding massive infrared dark clouds: A view by
  near-infrared dust extinction
  Context: Infrared Dark Clouds (IRDCs) harbor progenitors of high-mass stars.
Little is known of the parental molecular clouds of the IRDCs. Aims: We
demonstrate the feasibility of the near-infrared (NIR) dust extinction mapping
in tracing the parental molecular clouds of IRDCs at the distances of D = 2.5 -
8 kpc. Methods: We derive NIR extinction maps for 10 prominent IRDC complexes
using a color-excess mapping technique and NIR data from the UKIDSS/Galactic
Plane Survey. We compare the resulting maps to the 13CO emission line data, to
the 8 \mu m dust opacity data, and to the millimeter dust emission data. We
derive distances for the clouds by comparing the observed NIR source densities
to the Besancon stellar distribution model and compare them to the kinematic
distance estimates. Results: The NIR extinction maps provide a view to the IRDC
complexes over the dynamical range of Av = 2 - 40 mag, in spatial resolution of
30". The NIR extinction data correlate well with the 13CO data and probe a
similar gas component, but also extend to higher column densities. The NIR data
reveal a wealth of extended structures surrounding the dense gas traced by the
8 \mu m shadowing features and sub-mm dust emission, showing that the clouds
contain typically > 10 times more mass than traced by those tracers. The IRDC
complexes of our sample contain relatively high amount of high-column density
material, and their cumulative column density distributions resemble active
nearby star-forming clouds like Orion rather than less active clouds like
California. Conclusions: NIR dust extinction data provide a new powerful tool
to probe the mass distribution of the parental molecular clouds of IRDCs up to
the distances of D = 8 kpc. This encourages for deeper NIR observations of
IRDCs, because the sensitivity and resolution of the data can be directly
enhanced with dedicated observations.
