Water-Vapor Maser Disk at the Nucleus of the Seyfert 2 Galaxy NGC 3079

We observed a Seyfert 2/LINER galaxy NGC 3079 using the NRO 45-m telescope from 1992 to 2002, and the Very Long Baseline Array (VLBA) in 2001 December, at 22 GHz. The 45-m telescope detected weak red-shifted and systemic features as well as strong blue-shifted features. High spatial resolution observations with the VLBA indicated that the $\mathrm{H_2O}$ maser spots are distributed along the north-south direction, showing blue-shifted velocities in the north and red-shifted in the south. We propose a nearly edge-on molecular gas disk in the nuclear region of NGC 3079, whose radius varies from 0.4 to 1.2 pc and whose thickness is 0.3 pc. The disk is rotating at $V_\mathrm{rot} = 200 \hbox{--} 100 \,\mathrm{km} \,\mathrm{s}^{-1}$ , having a large velocity dispersion of $\Delta V \approx 50 \,\mathrm{km} \,\mathrm{s}^{-1}$ . The binding mass within the disk is $\sim 10^6 \,{{{M}_{\odot}}}$ . The disk is inclined such that the east side is nearest to us. The large luminosity of the maser emission and exceptionally strong blue-shifted features of NGC 3079 are probably the result of local shocks in the disk. Three continuum components are interpreted as jets emitted from the nucleus. The velocities of the components relative to the dynamical center of the masing disk are $(0.09 \hbox{--} 0.17) \,c,