Evidence for a geometrically thick self-gravitating accretion disk in NGC 3079

We have mapped, for the first time, the full velocity extent of the water maser emission in NGC 3079. The largely north-south distribution of emission, aligned with a kiloparsec-scale molecular disk, and the segregation of blueshifted and redshifted emission on the sky are suggestive of a nearly edge-on molecular disk on parsec scales. Positions and line-of-sight velocities of blueshifted and redshifted maser emission are consistent with a central mass of similar to2 x 10(6) M-circle dot enclosed within a radius of similar to0.4 pc. The corresponding mean mass density of 10(6.8) M-circle dot pc(-3) is suggestive of a central black hole, which is consistent with the detection of hard X-ray excess (20-100 keV) and an Fe Kalpha line from the nucleus. Because the rotation curve traced by the maser emission is flat, the mass of the parsec-scale disk is significant with respect to the central mass. Since the velocity dispersion of the maser features does not decrease with radius and constitutes a large fraction of the orbital velocity, the disk is probably thick and flared. The rotation curve and the physical conditions necessary to support maser emission imply a Toomre Q-parameter that is much less than1. Thus, the disk is most likely clumpy, and we argue that it is probably forming stars. Overall, the accretion disk in NGC 3079 stands in contrast to the compact, thin, warped, differentially rotating disk in the archetypal maser galaxy NGC 4258. We have also mapped radio continuum emission in the vicinity of the disk and identify a new, time-variable, nonthermal component ( E) that is not collinear with the previously imaged putative jet. Based on the large luminosity and the unusually steep spectrum (alpha < -2.1), we exclude a radio supernova as the progenitor of E. However, because its spectrum is consistent with an aging electron energy distribution, E might be a rapidly cooling remnant, which may indicate that the jet axis wobbles. Alternatively, considering its location, the component might mark a shock in a wide-angle outflow that is interacting with a dense ambient medium. In this context, masers at high latitudes above the disk, mapped in this and previous studies, may be tracing an inward extension of the kiloparsec-scale bipolar wide-angle outflow previously observed along the galactic minor axis.