2006A&A...450..569P

While low mass clouds have been relatively well studied, our picture of high-mass star formation remains unclear. Infrared Dark Clouds appear to be the long sought population of cold and dense aggregations with the potential of harbouring the earliest stages of massive star formation. Up to now there has been no systematic study on the temperature distribution, velocity fields, chemical and physical state toward this new cloud population. Knowing these properties is crucial for understanding the presence, absence and the very potential of star formation. The present paper aims at addressing these questions. We analyse temperature structures and velocity fields and gain information on their chemical evolution. We mapped the (J,K)=(1,1) and (2,2) inversion transitions of ammonia in 9 infrared dark clouds. Our observations allow the most reliable determination of gas temperatures in IRDCs to date. The gas emission is remarkably coextensive with the extinction seen at infrared wavelengths and with the submillimeter dust emission. Our results show that IRDCs are on average cold (T<20K) and have variations among the different cores. IRDC cores are in virial equilibrium, are massive (M>100M_☉), highly turbulent (1-3km/s) and exhibit significant velocity structure (variations around 1-2km/s over the cloud). We find an increasing trend in temperature from IRDCs with high ammonia column density to high mass protostellar objects and Ultracompact Hii regions, stages of early warm high-mass star formation. The linewidths of IRDCs are smaller than those observed in high mass protostellar objects and hot core/Ultracompact Hii regions. On basis of this sample, and by comparison of the ammonia gas properties within a cloud and between different clouds, we infer that while active star formation is not yet pervasive in most IRDCs, local condensations might collapse in the future or have already begun forming stars.