Abstract
The APEX Telescope Large Area Survey: The Galaxy (ATLASGAL) is an unbiased continuum survey of the inner Galactic disk at 870 \mu m. It covers +/- 60 deg in Galactic longitude and aims to find all massive clumps at various stages of high-mass star formation in the inner Galaxy, particularly the earliest evolutionary phases. We aim to determine properties such as the gas kinetic temperature and dynamics of new massive cold clumps found by ATLASGAL. Most importantly, we derived their kinematical distances from the measured line velocities. We observed the ammonia (J,K) = (1,1) to (3,3) inversion transitions toward 862 clumps of a flux-limited sample of submm clumps detected by ATLASGAL and extracted 13CO (1-0) spectra from the Galactic Ring Survey (GRS). We determined distances for a subsample located at the tangential points (71 sources) and for 277 clumps whose near/far distance ambiguity is resolved. Most ATLASGAL clumps are cold with rotational temperatures from 10-30 K. They have a wide range of NH3 linewidths, which by far exceeds the thermal linewidth, as well as a broad distribution of high column densities with an NH3 abundance in the range of 5 to 30 * 10^{-8}. We found an enhancement of clumps at Galactocentric radii of 4.5 and 6 kpc. The high detection rate (87%) confirms ammonia as an excellent probe of the molecular content of the massive, cold clumps revealed by ATLASGAL. A clear trend of increasing rotational temperatures and linewidths with evolutionary stage is seen for source samples ranging from 24 \mu m dark clumps to clumps with embedded HII regions. The survey provides the largest ammonia sample of high-mass star forming clumps and thus presents an important repository for the characterization of statistical properties of the clumps and the selection of subsamples for detailed, high-resolution follow-up studies.
Highlights
High-mass stars are essential for the evolution of galaxies because they energize the interstellar medium and release heavy elements, which are determining Galactic cooling mechanisms
We observed the ammonia (J, K) = (1, 1) to (3, 3) inversion transitions toward 862 clumps of a flux-limited sample of submm clumps detected by ATLASGAL and extracted 13CO (1−0) spectra from the Galactic Ring Survey (GRS)
Young massive stars evolving in dense cores within the clumps emit ultraviolet radiation that leads to the formation of ultracompact HII regions (UCHIIRs)
Summary
High-mass stars are essential for the evolution of galaxies because they energize the interstellar medium and release heavy elements, which are determining Galactic cooling mechanisms. The understanding of high-mass star formation is of great importance, only little is known about the early stages of the formation of massive stars in contrast to the well-established evolutionary sequence of isolated low-mass stars (André et al 2000). This is partly because of the difficult observational conditions that one encounters. Wood & Churchwell 1989b), which revealed their distribution and number Those regions were very helpful in tracing a more evolved stage of massive star formation in the Galaxy. Often UCHIIRs are associated with so-called hot molecular cores (Cesaroni et al 1992), which are possible precursors of UCHIIRs, called highmass protostellar objects (HMPOs) or massive young stellar
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