Molecular gas mass and far-infrared emission from distant luminous galaxies

Molecular line observations suggest the central few hundred parsecs of ultraluminous infrared galaxies have high mean gas densities, n(H 2 )∼3×10 3 to 10 4 cm -3 , 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