Temperature structures in Galactic center clouds

Direct evidence for gas heating via turbulence^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: ksimmer09@gmail.com
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 22 April 2016
Accepted: 5 July 2016

Abstract

The central molecular zone at the center of our Galaxy is the best template to study star formation processes under extreme conditions, similar to those in high-redshift galaxies. We observed on-the-fly maps of para-H₂CO transitions at 218 GHz and 291 GHz towards seven Galactic center clouds. From the temperature-sensitive integrated intensity line ratios of H₂CO(3_2,1 −2_2,0)/H₂CO(3_0,3 −2_0,2) and H₂CO(4_2,2 −3_2,1)/H₂CO(4_0,4 −3_0,3) in combination with radiative transfer models, we produce gas temperature maps of our targets. These transitions are sensitive to gas with densities of ~10⁵ cm^-3 and temperatures <150 K. The measured gas temperatures in our sources are all higher (>40 K) than their dust temperatures (~25 K). Our targets have a complex velocity structure that requires a careful disentanglement of the different components. We produce temperature maps for each of the velocity components and show that the temperatures of the components differ, revealing temperature gradients in the clouds. Combining the temperature measurements with the integrated intensity line ratio of H₂CO(4_0,4 −3_0,3)/H₂CO(3_0,3 −2_0,2), we constrain the density of this warm gas to 10⁴ −10⁶ cm^-3. We find a positive correlation between the line width of the main H₂CO lines and the temperature of the gas, direct evidence for gas heating via turbulence. Our data is consistent with a turbulence heating model with a density of n = 10⁵ cm^-3.