Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...413..542s&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 413, no. 2, p. 542-547.
Astronomy and Astrophysics
Astrophysics
63
Cosmic Rays, Interplanetary Medium, Molecular Gases, Starburst Galaxies, Gas Temperature, Hydrogen, Molecular Clouds
Scientific paper
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. We estimate the density of cosmic rays in the starburst galaxy M82 and calculate the temperature and density for the molecular clouds there. 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 to nonthermal radio emission 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. 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.
Allen Ronald J.
Heckman Timothy M.
Suchkov Anatolij
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