Massive accretion disks in galactic nuclei

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, while in lower luminosity systems, this material is probably concentrated by stellar bars and viscous accretion. In both cases, the nuclear region will be highly obscured by dust at visible wavelengths, forcing studies to longer wavelengths where the extinction is reduced. We review recent high resolution near infrared (HST-NICMOS) and mm-interferometric imaging of the dense gas and dust accretion disks in nearby luminous galactic nuclei. Since this nuclear ISM is the active ingredient for both starburst activity and a likely fuel for central AGNs, the nuclear accretion disks are critical to both the activity and the optical appearance of the nucleus. For a sample of 24 luminous galaxies imaged with NICMOS at 1-2 mu m, approximately 1/3 show nuclear point sources, indicating the existence of a central AGN or an intense starburst at less than or equal to 50 pc radius. Approximately 1/4 of the sample galaxies have apparent central dust disks. In the best studied ultraluminous IR galaxy, Arp 220, the 2 mu m imaging shows dust disks in both of the merging galactic nuclei and mm-CO line imaging indicates molecular gas masses similar to 10(9)M. for each disk. The two gas disks in Arp 220 are counterrotating and their dynamical masses are similar to 2x10(9)M., that is, only slightly larger than the gas masses. These disks have radii similar to 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 similar to 25-50% and mean densities of greater than or equal to 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 greater than or equal to 10 M. yr(-1) If this inflow persists to very small radii, it is enough to feed even the highest luminosity AGNs. (C)1999 COSPAR. Published by Elsevier Science Ltd.