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

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.

32 pages, 6 figures, accepted for publication by the Astrophysical Journal, for a pdf version with color figures, see http://www.astro.utoronto.ca/~muehle/papers/h2co1.pdf

References in corpus (5)

• The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation
• A 2 mm spectral line survey of the starburst galaxy NGC 253
• WFCAM, Spitzer-IRAC and SCUBA observations of the massive star forming region DR21/W75: I. The collimated molecular jets
• Detection of CO+ in the nucleus of M82
• Methanol detection in M82

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• Tracing high density gas in M 82 and NGC 4038
• Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde IV. The ALMA view of N113 and N159W in the LMC
• Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde V. The massive filament DR21
• Properties of dense molecular gas along the major axis of M 82
• Modelling Mechanical Heating in Star-Forming Galaxies: CO and 13CO Line Ratios as Sensitive Probes