Characterization of infrared dark clouds. NH3 observations of an absorption-contrast selected IRDC sample

Context. Despite increasing research in massive star formation, little is known about its earliest stages. Infrared dark clouds (IRDCs) are cold, dense and massive enough to harbour the sites of future high-mass star formation. But up to now, mainly small samples have been observed and analysed.
Aims: To understand the physical conditions during the early stages of high-mass star formation, it is necessary to learn more about the physical conditions and stability in relatively unevolved IRDCs. Thus, for characterising IRDCs studies of large samples are needed.
Methods: We investigate a complete sample of 218 northern hemisphere high-contrast IRDCs using the ammonia (1,1)- and (2,2)-inversion transitions.
Results: We detected ammonia (1,1)-inversion transition lines in 109 of our IRDC candidates. Using the data we were able to study the physical conditions within the star-forming regions statistically. We compared them with the conditions in more evolved regions which have been observed in the same fashion as our sample sources. Our results show that IRDCs have, on average, rotation temperatures of 15 K, are turbulent (with line width FWHMs around 2 km s^-1), have ammonia column densities on the order of 10¹⁴ cm^-2 and molecular hydrogen column densities on the order of 10²² cm^-2. Their virial masses are between 100 and a few 1000 M_⊙. The comparison of bulk kinetic and potential energies indicate that the sources are close to virial equilibrium.
Conclusions: IRDCs are on average cooler and less turbulent than a comparison sample of high-mass protostellar objects, and have lower ammonia column densities. Virial parameters indicate that the majority of IRDCs are currently stable, but are expected to collapse in the future.

Appendices A and B are available in electronic form at http://www.aanda.orgTables C.1-C.4 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/552/A40