2011A&A...536A..48K

Infrared dark clouds (IRDCs) harbor progenitors of high-mass stars. Little is known of the parental molecular clouds of IRDCs. 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 ≃2.5-8kpc. 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 µm 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. The NIR extinction maps provide a view of the IRDC complexes over the dynamical range of A_V≃2-40mag, 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 8µm 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 a 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. The 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 ∼8kpc. This encourages deeper NIR observations of IRDCs, because the sensitivity and resolution of the data can be directly enhanced with dedicated observations. In addition to mass distributions, the NIR data provide relatively reliable distance estimates.