Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies

New high resolution interferometer data of 10 IR ultraluminous galaxies showsthe molecular gas is in rotating nuclear rings or disks with radii 300 to 800pc. Most of the CO flux comes from a moderate-density, warm, intercloud mediumrather than self-gravitating clouds. Gas masses of ~ 5 x 10^9 Msun, 5 timeslower than the standard method are derived from a model of the molecular disks.The ratio of molecular gas to dynamical mass, is M_gas/M_dyn ~ 1/6 with amaximum ratio of gas to total mass surface density of 1/3. For the galaxiesVIIZw31, Arp193, and IRAS 10565+24, there is good evidence for rotatingmolecular rings with a central gap. In addition to the rotating rings a newclass of star formation region is identified which we call an ExtremeStarburst. They have a characteristic size of only 100 pc., about 10^9 Msun ofgas and an IR luminosity of ~3 x 10^11 Lsun. Four extreme starbursts areidentified in the 3 closest galaxies in the sample Arp220, Arp193 and Mrk273.They are the most prodigious star formation events in the local universe, eachrepresenting about 1000 times as many OB stars as 30 Doradus. In Arp220, the COand 1.3 mm continuum maps show the two ``nuclei'' embedded in a central ring ordisk and a fainter structure extending 3 kpc to the east, normal to the nucleardisk. There is no evidence that these sources really are the pre-merger nuclei.They are compact, extreme starburst regions containing 10^9 Msun of densemolecular gas and new stars, but no old stars. Most of the dust emission andHCN emission arises in the two extreme starbursts. The entire bolometricluminosity of Arp~220 comes from starbursts, not an AGN. In Mrk231, the diskgeometry shows that the molecular disk cannot be heated by the AGN; the far IRluminosity of Mrk~231 is powered by a starburst, not the AGN. (Abridged)