Formaldehyde as a Tracer of Extragalactic Molecular Gas I. Para-H_2CO Emission From M 82

TL;DR

This study demonstrates that para-formaldehyde (p-H_2CO) can serve as an effective molecular thermometer for extragalactic molecular gas, revealing higher kinetic temperatures than previously estimated in M 82's starburst region.

Contribution

It introduces p-H_2CO as a novel tracer for density and temperature in external galaxies and provides detailed measurements of these parameters in M 82 using large velocity gradient modeling.

Findings

p-H_2CO acts as a direct molecular thermometer.

Derived gas densities are around 7 x 10^3 cm^-3.

Kinetic temperatures are approximately 200 K, higher than dust and NH_3 estimates.

Abstract

Using the IRAM 30-m telescope and the 15-m JCMT, we explore the value of para-formaldehyde (p-H_2CO) as a tracer of density and temperature of the molecular gas in external galaxies. The target of our observations are the lobes of the molecular ring around the center of the nearby prototypical starburst galaxy M 82. It is shown that p-H_2CO provides one of the rare direct molecular thermometers. Reproducing the measured line intensities with a large velocity gradient (LVG) model, we find densities of n_H2 ~ 7 10^3 cm^-3 and kinetic temperatures of T_kin ~ 200 K. The derived kinetic temperature is significantly higher than the dust temperature or the temperature deduced from ammonia (NH_3) lines, but our results agree well with the properties of the high-excitation component seen in CO. We also present the serendipitous discovery of the 4_2 - 3_1 line of methanol (CH_3OH) in the northeastern lobe, which shows -- unlike CO and H_2CO -- significantly different line intensities in the two lobes.