Massive Accretion Disks

1. Abstract Recent high resolution near infrared (HST-NICMOS) and mm-interferometric imaging have revealed dense gas and dust accretion disks in nearby ultra-luminous galactic nuclei. In the best studied ultraluminous IR galaxy, Arp 220, the 2µm imaging shows dust disks in both of the merging galactic nuclei and mm-CO line imaging indicates molecular gas masses ∼10 9 M ⊙ for each disk. The two gas disks in Arp 220 are counterrotating and their dynamical masses are ∼ 2x10 9 M ⊙ , that is, only slightly larger than the gas masses. These disks have radii ∼100 pc and thickness 10-50 pc. The high brightness temperatures of the CO lines indicate that the gas in the disks has area filling factors ∼25-50% and mean densities of ≥ 10 4 cm −3. Within these nuclear disks, the rate of massive star formation is undoubtedly prodigious and, given the high viscosity of the gas, there will also be high radial accretion rates, perhaps ≥10 M ⊙ yr −1. If this inflow persists to very small radii, it is enough to feed even the highest luminosity AGNs. In the luminous infrared galaxies, very large masses of interstellar matter have been concentrated in the galactic nuclei at radii less than 300 pc as a result of galactic merging. Due to its relatively large volume-filling factor and ability to cool rapidly, the ISM is much more dissipative than the stars. This dissipation of rotational energy and angular momentum will concentrate the ISM in the merging galactic nucleus. The resultant high density of ISM in the nucleus then becomes the active ingredient for luminosity generation – via enormously elevated rates of star formation or by feeding a pre-existing massive black hole. There new evidence for massive