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CO excitation and H2 masses of infrared-luminous galaxies
Radford, Simon J. E.; Solomon, P. M.; Downes, D.
AA(Institut de Radioastronomie Millimetrique, Saint-Martin-d'Heres, France), AB(Institut de Radioastronomie Millimetrique, Saint-Martin-d'Heres, France), AC(Grenoble, Observatoire, France; Max-Planck-Institut für Radioastronomie, Bonn, Germany; New York, State University, Stony Brook)
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 368, Feb. 10, 1991, p. L15-L18. (ApJL Homepage)
Publication Date:
NASA/STI Keywords:
Carbon Monoxide, Galactic Radiation, Gas Density, Hydrogen, Infrared Sources (Astronomy), Interstellar Gas, Molecular Clouds, Molecular Excitation, Brightness Temperature, Density Measurement, Mass To Light Ratios, Milky Way Galaxy, Quasars
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The CO(2-1) and CO(1-0) emission from four infrared-luminous galaxies, Arp 193, Arp 220, Mrk 231, and VII Zw 31, was mapped with the IRAM 30 m telescope. These maps show the molecular gas is concentrated in the central regions. The CO(2-1)/CO(1-0) brightness temperature ratio for these galaxies is low, 0.6-0.75, indicating the CO is subthermally excited in regions of moderate H2 density, roughly 400/cu cm. The intrinsic CO(1-0) brightness temperatures are inferred to be between 6 and 13 K, even if the gas kinetic temperatures are much higher. For these galaxies, the H2 mass-to-CO luminosity ratio is similar to that measured for giant molecular clouds in the Milky Way molecular ring.

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