Rotating nuclear rings and extreme starbursts in ultraluminous galaxies

New CO interferometer data show that the molecular gas in infrared ultralum inous galaxies is in rotating nuclear disks or rings. The CO maps yield dis k radii, kinematic major axes, rotation speeds, enclosed dynamical masses, and gas masses. The CO brightness temperatures, the double-peaked CO line p rofiles, the limits on thermal continuum flux from dust, and the constraint that the gas mass must be less than the dynamical mass all indicate that t he CO lines are subthermally excited and moderately opaque (tau = 4 to 10). We fit kinematic models in which most of the CO flux comes from a moderate -density warm intercloud medium, rather than from self-gravitating clouds. Typical ring radii are 300 to 800 pc. We derive gas masses not from a stand ard CO-to-mass ratio, but from a model of radiative transfer through subthe rmally excited CO in the molecular disks. This model yields gas masses of s imilar to 5 x 10(9) M., similar to 5 times lower than the standard method, and a ratio M-gas/L-CO' approximate to 0.8 M. (K km s(-1) pc(2))(-1). In th e nuclear disks, we derive a ratio of gas to dynamical mass of M-gas/M-dyn approximate to 1/6, and a maximum ratio of gas to total mass surface densit y, mu/mu(tot), of 1/3. For the galaxies VII Zw 31, Arp 193, and IRAS 10565 + 2448, the CO position-velocity diagrams provide good evidence for rotatin g molecular rings with a central gap. In addition to the rotating central r ings or disks, a new class of star formation region is identified, which we call an extreme starburst. These have a characteristic sizes of only 100 p c, with about 10(9) M. of gas and an IR luminosity of approximate to 3 x 10 (11) L. from recently formed OB stars. Four extreme starbursts are identifi ed in the 3 closest galaxies in the: sample, including Arp 220, Arp 193, an d Mrk 273. These are the most prodigious star formation events in the local universe, each representing about 1000 times as many OB stars as 30 Doradu s. In Mrk 231, the CO (2-1) velocity diagram along the line of nodes shows a 1." 2 diameter inner disk and a 3 " diameter outer disk. The narrow CO li ne width, the single-peak line profile, the equality of the major and minor axes, and the observed velocity gradients all imply that the molecular dis k is nearly face-on, yielding low optical and UV extinction to the active g alactic nucleus (AGN). Such a geometry means that the molecular disk cannot be heated by the AGN; the far-infrared (FIR) luminosity of Mrk 231 is powe red by a starburst, not the AGN. In Mrk 273, the CO (1-0) maps show long st reamers of radius 5 kpc (7 ") with velocity gradients north-south, and a nu clear disk of radius 400 pc (0." 6) with velocity gradients east-west. The nuclear disk contains a bright CO core of radius 120 pc (0." 2). In Arp 220 , the CO and 1.3 mm continuum maps show the two "nuclei" embedded in a cent ral ring or disk at P.A. 50 degrees and a fainter structure extending 7 " ( 3 kpc) to the east, normal to the nuclear disk. Models of the CO and dust f lux indicate that the two K-band sources contain high-density gas, with n(H -2) = 2 x 10(4) cm(-3) There is no evidence that these sources really are t he premerger nuclei. They are more likely to be compact extreme starburst r egions, containing 10(9) M. of dense molecular gas and new stars, but no ol d stars. Most of the HCN emission arises in the two nuclei. The luminosity- to-mass ratios for the CO sources in Arp 220 are compatible with the early phases of compact starbursts. There is a large mass of molecular gas curren tly forming stars with plenty of ionizing photons, and no obvious AGN. The entire bolometric luminosity of Arp 220 comes from starbursts, not an A GN. The CO maps show that the gas in ultraluminous IR galaxies is in extend ed disks that cannot intercept all the power of central AGNs, even if they exist. We conclude that in ultraluminous IR galaxies-even in Mrk 231, which hosts a quasar-the FIR luminosity is powered by extreme starbursts in the molecular rings or disks, not by dust-enshrouded quasars.