H2O Megamasers and Black Holes

Abstract

AbstractTwenty-one H2O masers have been identified in the nuclei of active galaxies. The detection rate is about 7 percent. Very Long Baseline Interferometric data of four of these maser systems (NGC 4258, NGC 1068, NGC 4945, and NGC 3079) show evidence of Keplerian disks on a subparsec scale. The best example is NGC 4258. There the masers trace a thin warped disk in nearly perfect circular Keplerian motion. If the apparent thinness is indicative of hydrostatic equilibrium, then the kinetic temperature must be less than 1000K, the magnetic field less than 200 mG, and the mass accretion rate less than about α10-3 M⊙/yr, where a is the viscosity parameter. From direct Zeeman measurements the toroidal magnetic field strength is less than 300 mG. The proper motions of the systemic maser feature imply a distance to the galaxy of 7.3 ± 0.3 Mpc. The high-velocity features show no detectable proper motions or accelerations, which confirms the model of circular Keplerian motion, and puts severe constraints on any alternative model. A feature in the systemic group flared to 20 Jy, offering opportunities to study the physics of the maser emission.