A Census of the High-density Molecular Gas in M82

• Published in: The Astrophysical journal Vol. 722; no. 1; pp. 668 - 681
• Main Authors: Naylor, B. J, Bradford, C. M, Aguirre, J. E, Bock, J. J, Earle, L, Glenn, J, Inami, H, Kamenetzky, J, Maloney, P. R, Matsuhara, H, Nguyen, H. T, Zmuidzinas, J
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 10.10.2010; IOP
• Subjects: ABUNDANCE; Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BINARY STARS; COSMIC RADIATION; Earth, ocean, space; ELECTROMAGNETIC RADIATION; ERUPTIVE VARIABLE STARS; EVEN-EVEN NUCLEI; Exact sciences and technology; GALAXIES; HYDROCYANIC ACID; HYDROGEN COMPOUNDS; INORGANIC ACIDS; INORGANIC COMPOUNDS; IONIZING RADIATIONS; ISOTOPES; LIGHT NUCLEI; MILKY WAY; NUCLEI; RADIATIONS; STABLE ISOTOPES; STAR CLUSTERS; STARS; SULFUR 34; SULFUR ISOTOPES; SUPERNOVAE; ULTRAVIOLET RADIATION; VARIABLE STARS; galaxies: starburst; Supernovae; High density; Starburst galaxies; Molecular gas; Cosmic radiation; Massive stars; galaxies: ISM; techniques: spectroscopic; Initial mass function; galaxies: abundances; Intensity; Stellar formation region; galaxies: individual (M82); Abundance; instrumentation: spectrographs; Star clusters; Milky Way; Hydrogen cyanides; Upper bound; Ultraviolet irradiation; Low-mass stars; Stellar population; Dense gas; Interstellar matter
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/722/1/668

We present a three-pointing study of the molecular gas in the starburst nucleus of M82 based on 190-307 GHz spectra obtained with Z-Spec at the Caltech Submillimeter Observatory. We present intensity measurements, detections, and upper limits, for 20 transitions, including several new detections of CS, HNC, C{sub 2}H, H{sub 2}CO, and CH{sub 3}CCH lines. We combine our measurements with previously published measurements at other frequencies for HCN, HNC, CS, C{sup 34}S, and HCO{sup +} in a multi-species likelihood analysis constraining gas mass, density and temperature, and the species' relative abundances. We find some (1.7-2.7) x 10{sup 8} M{sub sun} of gas with n{sub H{sub 2}} between (1-6) x 10{sup 4} cm{sup -3} and T > 50 K. While the mass and temperature are comparable to values inferred from mid-J CO transitions, the thermal pressure is a factor of 10-20 greater. The molecular interstellar medium is largely fragmented and is subject to ultraviolet irradiation from the star clusters. It is also likely subject to cosmic rays and mechanical energy input from the supernovae, and is warmer on average than the molecular gas in the massive star formation (SF) regions in the Milky Way. The typical conditions in the dense gas in M82's central kiloparsec appear unfavorable for further SF; if any appreciable stellar populations are currently forming, they are likely biased against low-mass stars, producing a top-heavy initial mass function.