High-resolution CO observations of the ultraluminous infrared galaxy Markarian 231

We have mapped the luminous infrared galaxy Mrk 231 in the CO (J = 1 --> 0) and (J = 2 --> 1) transitions with the Owens Valley millimeter array, The maps show a very compact emission region with diameter 1 ''.0 (840 pc). Although the CO emission is only slightly resolved in the lower frequency CO line (J = 1 --> 0), it is clearly resolved in the 0 ''.8 J = 2 --> 1 beam. Both maps reveal an east-west velocity gradient aligned with the major axis of the CO emission distribution. We analyze in detail the derivation of dynamical and molecular gas masses from CO observations for a variety of geometries and physical conditions (including optically thin CO emission, spherical distributions with uniform and clumpy gas, and self-gravitating and externally bound disks). In Mrk 231 the molecular gas is not likely to be distributed spherically about the nucleus, since this hypothesis forces the derived gas mass to be larger than the dynamical mass (except for a very limited parameter space). We therefore propose that the gas is distributed in a disk. The axis of this disk is constrained to be within 59 degrees of our line of sight unless the Galactic M(gas)/L'(CO) ratio is overestimating the gas mass by over a factor of 3. This geometry is consistent with the modest extinctions indicated from optical and near-infrared spectroscopy. The disk must be within 22 degrees of face-on if the molecular gas mass is equal to the estimate obtained using the Galactic conversion ratio. The similarity of the CO line brightness temperature and the far-infrared dust color temperature suggests that the molecular disk is mostly filled with dense gas clouds. This massive, viscous disk can account for the sustained, high accretion rate required to power the ultraluminous nucleus.