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

TL;DR

This study characterizes the physical conditions of molecular gas in W49A, revealing warm, dense gas similar to starburst galaxies, likely heated by shocks and stellar winds, providing insights into starburst conditions within our Galaxy.

Contribution

It provides detailed measurements of temperature and density in W49A, demonstrating starburst-like conditions in a Galactic star-forming region for the first time.

Findings

W49A contains warm (>100 K) and dense (>10^5 cm^-3) molecular gas.

The gas mass is estimated between 2x10^4 and 2x10^5 solar masses.

Heating is likely caused by shocks and stellar winds, not cosmic rays or X-ray irradiation.

Abstract

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. 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. We probe the temperature and density structure of W49A using H_2CO and HCN line ratios over a 2'x2' (6.6x6.6 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. Comparing the observed line intensity ratios with non-LTE radiative transfer models, our results reveal an extended region (about 1'x1', equivalent to ~3x3 pc at the distance of W49A) of warm (> 100 K) and dense (>10^5 cm^-3) molecular gas, with a mass of 2x10^4 - 2x10^5 M_Sun (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. 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.