Extended warm and dense gas towards W49A: starburst conditions in our Galaxy?

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: nagy@astro.rug.nl
² SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK
⁴ Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada

Received: 5 December 2011
Accepted: 28 March 2000

Abstract

Context. The star formation rates in starburst galaxies are orders of magnitude higher than in local star-forming regions, and the origin of this difference is not well understood.

Aims. We use sub-mm spectral line maps to characterize the physical conditions of the molecular gas in the luminous Galactic star-forming region W49A and compare them with the conditions in starburst galaxies.

Methods. We probe the temperature and density structure of W49A using H₂CO and HCN line ratios over a 2′ × 2′ (6.6 × 6.6 pc) field with an angular resolution of 15′′ (~0.8 pc) provided by the JCMT Spectral Legacy Survey. We analyze the rotation diagrams of lines with multiple transitions with corrections for optical depth and beam dilution, and estimate excitation temperatures and column densities.

Results. Comparing the observed line intensity ratios with non-LTE radiative transfer models, our results reveal an extended region (about 1′ × 1′, equivalent to ~3 × 3 pc at the distance of W49A) of warm (>100 K) and dense (> 10⁵ cm^-3) molecular gas, with a mass of 2 × 10⁴ − 2 × 10⁵ M_⊙ (by applying abundances derived for other regions of massive star-formation). These temperatures and densities in W49A are comparable to those found in clouds near the center of the Milky Way and in starburst galaxies.

Conclusions. The highly excited gas is likely to be heated via shocks from the stellar winds of embedded, O-type stars or alternatively due to UV irradiation, or possibly a combination of these two processes. Cosmic rays, X-ray irradiation and gas-grain collisional heating are less likely to be the source of the heating in the case of W49A.