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Using the IRAM 30-m telescope and the 15-m JCMT, we explore the value ofpara-formaldehyde (p-H_2CO) as a tracer of density and temperature of themolecular gas in external galaxies. The target of our observations are thelobes of the molecular ring around the center of the nearby prototypicalstarburst galaxy M 82. It is shown that p-H_2CO provides one of the rare directmolecular thermometers. Reproducing the measured line intensities with a largevelocity gradient (LVG) model, we find densities of n_H2 ~ 7 10^3 cm^-3 andkinetic temperatures of T_kin ~ 200 K. The derived kinetic temperature issignificantly higher than the dust temperature or the temperature deduced fromammonia (NH_3) lines, but our results agree well with the properties of thehigh-excitation component seen in CO. We also present the serendipitousdiscovery 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 intensitiesin the two lobes.

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