Extended Molecular Gas in FSC 10214+4724

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Abstract
We report high resolution imaging of the CO (3-2) emission in the luminous infrared source FSC 10214+4724 at z = 2.2853. Maps at resolutions of 23 × 30 and 45-90 km s-1 show the CO emission peak coinciding with those of Hα and radio continuum. Two components are identified in the CO emission: an unresolved core and an extended source (~19 × 44)—elongated southeast-northwest. The implied size is 9 × 24 kpc for H0 = 75 km s-1 Mpc-1 and q0 = 0.5. The extent of the CO emission is significantly greater than that of either the optical or the radio continuum (~1'') and the CO extent varies between the line core and the wings. If the gravitational lensing is occuring in FSC 10214+4724 (as has been suggested based on the near infrared morphology), the magnification factor for the CO emission is likely to be lower than for the near-infrared and the observed CO extent is probably an upper limit to the true source size. The mass of molecular gas implied by the observed CO line flux is ~2 × 1011 M☉, assuming no lens magnification. In order that the derived gas mass does not exceed the dynamical mass, the gas would have to be in a nearly face-on (i < 20°) disk. However, this configuration appears inconsistent with the elongated morphology of the CO (which suggests an inclined disk or an interacting galaxy system). We suggest that the "dynamical" mass might be reconciled with the derived gas mass if substantial support for the gas is provided by radiation pressure from the nucleus of FSC 10214+4724. For the observed ratio of infrared luminosity-to-gas-mass (103 L☉ M☉−1), we find that a column density ≤2.4 × 1022 H2 cm-2 (AV ~ 24) could be supported; this is consistent with the average gas column that is observed. The very high luminosity-to-mass ratio strongly also favors a nonthermal origin for the luminosity since a starburst would require consumption of the entire ISM on a timescale significantly shorter than the dynamical times (2 × 107-2 × 108 yr) estimated from the CO data. These conclusions are not critically dependent on the presence or abscence of gravitational lensing since the molecular emission, the far-infrared flux, and the "dynamical" mass are similarly effected and the ratios therefore remained almost unchanged.