Doppler Shift Asymmetry in High‐Velocity Maser Emission from Shocks in Circumnuclear Disks

SHORT VERSION: The rapidly rotating, masing circumnuclear disk in the centralsub-parsec region of the galaxy NGC 4258 is remarkably circular and Keplerian,yet a striking asymmetry appears in the maser spectrum: the red-shifted, high-velocity sources are much more numerous and significantly more intense than theblue-shifted ones. A similar strong asymmetry appears also in the recentlydiscovered, masing, circumnuclear disks in NGC 1068 and NGC 4945, thussuggesting it may be a general phenomenon. We show that the observed Doppler-shift asymmetry can naturally arise due tospiral shocks in circumnuclear disks, independent of the existence of a warp inthe disk or the azimuthal direction to the observer. The high velocities ofthese features reflect the rotational velocities in the disk, and have nothingto do with the shock speed. In NGC 4258 - the currently most well-definedmasing disk - the proposed scenario can also account for the intriguingclustering of the high-velocity maser spots in distinct clumps, the restrictedspatial distribution of the low-velocity sources, and the dip in the maserspectrum at the systemic velocity of the disk. In this case we infer a diskmass of ~10E4 M_sun and a mass accretion rate of order ~7E-3 M_sun/year, whichmay be consistent with an advection-dominated accretion flow. The model isconsistent with the observed Keplerian rotation, and introduces only negligiblecorrections to the previously derived black hole mass and galaxy distance. Predictions include slow systematic drifts in the velocity and position ofall the high-velocity features, and the existence of circumnuclear disks whichare delineated only by high-velocity maser emission.Comment: 44 pages, Submitted to Ap