Cosmic-ray-dominated dense molecular gas in normal and starburst galaxies

Abstract

view Abstract Citations (90) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cosmic-Ray--dominated Dense Molecular Gas in Normal and Starburst Galaxies Suchkov, Anatolij ; Allen, Ronald J. ; Heckman, Timothy M. Abstract We explore the implications of the hypothesis that the various phases of the interstellar medium (ISM) in starburst galaxies are approximately in pressure equilibrium, and that the heating of molecular clouds is due mainly to cosmic rays. This model permits the unique determination of the density and temperature of the molecular gas, provided the cosmic-ray density is known. Using two independent techniques, we estimate the density of cosmic rays in the starburst galaxy M82 and calculate the temperature and density for the molecular clouds there. The predicted parameters turn out to be in good agreement with the values derived from various observations. The model of cosmic-ray heated molecular gas is also capable of accounting for the observed temperatures of molecular gas in the disk of our Galaxy. This suggests that, although UV photons may well contribute to gas heating in the immediate neighborhood of hot stars, cosmic rays appear to be the prime source of molecular cloud heating in the general ISM both for normal and for starburst galaxies. This model predicts that the ratio I_CO_/T_v_ of CO emission (which is a measure of molecular cloud kinetic temperature) to nonthermal radio emission (which is a measure of the amount of cosmic rays along the line of sight) is large for young star-forming regions in galactic disks and small for galactic centers as compared to the average value inferred for galactic disks (if not viewed edge-on). We argue that this ratio depends on the age of a starburst region, and therefore can be used as an age diagnostic of starbursts and galactic disk star-forming regions. In particular, we predict that I_CO_/T_v_ is larger for younger starbursts. Publication: The Astrophysical Journal Pub Date: August 1993 DOI: 10.1086/173023 Bibcode: 1993ApJ...413..542S Keywords: Cosmic Rays; Interplanetary Medium; Molecular Gases; Starburst Galaxies; Gas Temperature; Hydrogen; Molecular Clouds; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M82; GALAXIES: ISM; GALAXIES: STARBURST; ISM: COSMIC RAYS; ISM: MOLECULES full text sources ADS | data products SIMBAD (1)