ATLASGAL – Ammonia observations towards the southern Galactic plane ^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mwienen@mpifr-bonn.mpg.de
² School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK
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 Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia
⁶ Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile

Received: 22 April 2015
Accepted: 5 August 2017

Abstract

Context. The initial conditions of molecular clumps in which high-mass stars form are poorly understood. In particular, a more detailed study of the earliest evolutionary phases is needed. The APEX Telescope Large Area Survey of the whole inner Galactic disk at 870 μm, ATLASGAL, has therefore been conducted to discover high-mass star-forming regions at different evolutionary phases.

Aims. We derive properties such as velocities, rotational temperatures, column densities, and abundances of a large sample of southern ATLASGAL clumps in the fourth quadrant.

Methods. Using the Parkes telescope, we observed the NH₃ (1, 1) to (3, 3) inversion transitions towards 354 dust clumps detected by ATLASGAL within a Galactic longitude range between 300° and 359° and a latitude within ± 1.5°. For a subsample of 289 sources, the N₂H⁺ (1–0) line was measured with the Mopra telescope.

Results. We measured a median NH₃ (1, 1) line width of ~ 2 km s^-1, rotational temperatures from 12 to 28 K with a mean of 18 K, and source-averaged NH₃ abundances from 1.6 × 10^-6 to 10^-8. For a subsample with detected NH₃ (2, 2) hyperfine components, we found that the commonly used method to compute the (2, 2) optical depth from the (1, 1) optical depth and the (2, 2) to (1, 1) main beam brightness temperature ratio leads to an underestimation of the rotational temperature and column density. A larger median virial parameter of ~ 1 is determined using the broader N₂H⁺ line width than is estimated from the NH₃ line width of ~ 0.5 with a general trend of a decreasing virial parameter with increasing gas mass. We obtain a rising NH₃ (1, 1)/N₂H⁺ line-width ratio with increasing rotational temperature.

Conclusions. A comparison of NH₃ line parameters of ATLASGAL clumps to cores in nearby molecular clouds reveals smaller velocity dispersions in low-mass than high-mass star-forming regions and a warmer surrounding of ATLASGAL clumps than the surrounding of low-mass cores. The NH₃ (1, 1) inversion transition of 49% of the sources shows hyperfine structure anomalies. The intensity ratio of the outer hyperfine structure lines with a median of 1.27 ± 0.03 and a standard deviation of 0.45 is significantly higher than 1, while the intensity ratios of the inner satellites with a median of 0.9 ± 0.02 and standard deviation of 0.3 and the sum of the inner and outer hyperfine components with a median of 1.06 ± 0.02 and standard deviation of 0.37 are closer to 1.