2007ApJ...671.1579M

Using the IRAM 30 m telescope and the 15 m JCMT, we explore the value of paraformaldehyde (p-H₂CO) 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 M82. It is shown that p-H₂ CO 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∼7x10³/cm3 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 (NH₃) 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 4₂⟶3₁ line of methanol (CH₃OH) in the northeastern lobe, which shows–unlike CO and H₂CO–significantly different line intensities in the two lobes.