VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068

We have made the first VLBI synthesis images of the H2O maser emissionassociated with the central engine of the Seyfert galaxy NGC 1068. Emissionextends about +/-300 km/s from the systemic velocity. Images withsubmilliarcsecond angular resolution show that the red-shifted emission liesalong an arc to the northwest of the systemic emission. (The blue-shiftedemission has not yet been imaged with VLBI.) Based on the maser velocities andthe relative orientation of the known radio jet, we propose that the maseremission arises on the surface of a nearly edge-on torus, where physicalconditions are conducive to maser action. The visible part of the torus isaxially thick, with comparable height and radius. The velocity field indicatessub-Keplerian differential rotation around a central mass of about 1e7 Msunthat lies within a cylindrical radius of about 0.65 pc. The estimatedluminosity of the central engine is about 0.5 of the Eddington limit. There isno detectable compact radio continuum emission near the proposed center of thetorus (T_B< 5e6 K on size scales of about 0.1 pc), so that the observedflat-spectrum core cannot be direct self-absorbed synchrotron radiation.