Properties of the Molecular Gas in Starburst Galaxies and AGN

TL;DR

This study uses formaldehyde line observations to accurately determine the physical properties of dense molecular gas in starburst galaxy M82, revealing temperature and density details that differ from standard tracers and suggesting asymmetry in molecular lobes.

Contribution

It demonstrates the effectiveness of formaldehyde as a molecular thermometer and density tracer, providing a new method to study molecular gas in external galaxies independently of traditional tracers.

Findings

Formaldehyde lines effectively constrain gas temperature and density.

Results align with previous CO studies of high-excitation gas.

Evidence of asymmetry between molecular lobes in M82.

Abstract

There is growing evidence that the properties of the molecular gas in the nuclei of starburst galaxies and in AGN may be very different from those seen in Galactic star forming regions and that a high kinetic temperature in the molecular gas may lead to a non-standard initial mass function in the next generation of stars. Unfortunately, among the fundamental parameters derived from molecular line observations, the kinetic temperature of the molecular gas in external galaxies is often not well determined due to a lack of suitable tracer molecules. We discuss the diagnostic power of selected transition lines of formaldehyde (H_2CO), which can be used as a molecular thermometer as well as an excellent tracer of the molecular gas density. As a proof of concept, we present the results of our multi-transition line study of the H_2CO emission from the prototypical starburst galaxy M82. Using our large velocity gradient model, we tightly constrain the physical properties of the dense gas in the prominent molecular lobes, completely independent of the standard "cloud thermometer" ammonia (NH_3) or other molecular tracers. Our results agree well with the properties of the high-excitation molecular gas component found in the most comprehensive CO studies. Our observations also indicate that there may be an asymmetry between the two molecular lobes.