Gas and Dust in Ultraluminous Galactic Nuclei

Abstract

Millimeter-wave interferometry has clearly shown the existence of enormous masses (10^9 – 10^(10) M⨀) of molecular gas concentrated in the nuclear regions (R < 500 pc) of many luminous and ultraluminous infrared galaxies. In these systems, molecular gas is an obvious source of fuel for nuclear starbursts and active galactic nuclei (AGN). In several ultraluminous systems (e.g., Arp 220 and Mrk 231), there now exists CO (2-1) interferometry at < 1″ resolution which reveals for the first time extremely dense, gaseous accretion disks on the scale 50–300 pc. Based on the low velocity dispersion of the molecular gas in the nuclear disks, we believe them to be extremely thin (10–50 pc). In addition, high brightness temperatures in the CO lines (10–20 K) imply that these disks are nearly uniformly filled with a continuous gas distribution, rather than being relatively isolated, self-gravitating GMCs. Although the gas is ‘uniformly’ distributed, the gas densities must be high, > 10^4 cm^(-3). When viewed near the plane of the disk, the central AGNs, if they exist, will be totally obscured at optical and near infrared wavelengths. In Mrk 231, our line of sight is probably within 60° of the disk axis, but in Arp 220 the disk is closer to edge-on. In fact, recent near infrared imaging of Arp 220 with the NICMOS camera on the HST reveals totally opaque dust disks embedded within the central star clusters of both nuclei.