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Molecular gas mass and far-infrared emission from distant luminous galaxies
Downes, D.; Solomon, P. M.; Radford, S. J. E.
AA(Inst. de Radioastronomie Millimetrique, Saint-Martin-d'Heres, France), AB(New York State University, Stony Brook), AC(Inst. de Radioastronomie Millimetrique, Saint-Martin-d'Heres, France)
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 414, no. 1, p. L13-L16. (ApJL Homepage)
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NASA/STI Keywords:
Emission Spectra, Galactic Structure, Interplanetary Gas, Mass Distribution, Molecular Gases, Astronomical Models, Carbon Monoxide, Infrared Astronomy, Luminosity
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Molecular line observations suggest the central few hundred parsecs of ultraluminous IR galaxies have high mean gas densities, of about 3000 to 10,000/cu cm, unlike the centers of normal galaxies and very different from the disks of spiral galaxies. The CO line emission may not trace an ensemble of gravitationally bound gas clouds but instead a medium bound by the total potential of the Galactic center (gas and stars). This means the CO luminosity no longer measures gas mass alone, as in normal galaxies, but instead the geometric mean of the gas mass and the dynamical mass. In practice, molecular gas dominates the dynamical masses of the centers of ultraluminous galaxies, which suggests gas masses estimated from CO luminosities are basically correct. We present a model in which the 100/micron radiation from ultraluminous galaxies is optically thick and the CO line flux is proportional to the 100-micron flux, with S(CO)Delta(V)/S(100 microns) = 2-4 km/s. Our measurements of CO flux from 35 ultraluminous galaxies support this model of optically thick dust at 100 microns, providing additional evidence that the dust mass is high and the gas mass is a large fraction of the dynamical mass. We derive a relation between the M(gas)/L-prime(CO) ratio and the luminosity distance that we apply to Arp 220.

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