Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mwienen@mpifr-bonn.mpg.de
² Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile
³ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Dublin Institute of Advanced Studies, Fitzwilliam Place 31, Dublin 2, Ireland
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁶ Laboratoire AIM, CEA/IRFU – CNRS/INSU – Université Paris Diderot, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 16 September 2011
Accepted: 26 May 2012

Abstract

Context. The APEX Telescope Large Area Survey: the GALaxy (ATLASGAL) is an unbiased continuum survey of the inner Galactic disk at 870 μm. It covers  ±60° 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.

Aims. 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.

Methods. 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.

Results. Most ATLASGAL clumps are cold with rotational temperatures from 10−30 K with a median of 17 K. They have a wide range of NH₃ linewidths (1−7 km s^-1) with 1.9 km s^-1 as median, which by far exceeds the thermal linewidth, as well as a broad distribution of high column densities from 10¹⁴ to 10¹⁶ cm^-2 (median of 2 × 10¹⁵ cm^-2) with an NH₃ abundance in the range of 5 to 30 × 10^-8. ATLASGAL sources are massive,  ≳100 M_⊙, and a fraction of clumps with a broad linewidth is in virial equilibrium. We found an enhancement of clumps at Galactocentric radii of 4.5 and 6 kpc. The comparison of the NH₃ lines as high-density probes with the GRS ¹³CO emission as low-density envelope tracer yields broader linewidths for ¹³CO than for NH₃. The small differences in derived clump velocities between NH₃ (representing dense core material) and ¹³CO (representing more diffuse molecular cloud gas) suggests that the cores are essentially at rest relative to the surrounding giant molecular cloud.

Conclusions. 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 μm 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.