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

The rapidly rotating, masing circumnuclear disk in the central subparsec region of the galaxy NGC 4258 is remarkably circular and Keplerian, yet a striking asymmetry appears in the maser spectrum: the redshifted, high-velocity sources are much more numerous and significantly more intense than the blueshifted ones. A similar strong asymmetry also appears in the recently discovered, masing, circumnuclear disks in NGC 1068 and NGC 4945, thus suggesting it may be a general phenomenon. We show that the observed Doppler shift asymmetry can naturally arise due to spiral shocks in circumnuclear disks. We argue that population inversion can largely be quenched in these systems because of IR photon trapping, and that the high-velocity maser emission originates within thin slabs of postshock gas, where the physical conditions are conducive to maser action. The high-velocity masers with the longest gain paths appear where the line of sight is tangent to shock fronts. Since the spirals have a trailing geometry due to the action of differential rotation, the locations of the masers make the blueshifted radiation travel through a column of noninverted gas that maintains close velocity coherence with the maser source, where absorption occurs. The resulting asymmetry in the high-velocity maser spectrum, where the redshifted emission appears systematically stronger, is independent of the existence of a warp in the disk or the azimuthal direction to the observer, and is insensitive to small distortions in the velocity field in the disk. The high velocities of these features reflect the rotational velocities in the disk and have nothing to do with the shock speed. The low-velocity emission arises within a narrow annulus near the inner edge of the disk, where direct irradiation by a central source may provide the energy that ultimately powers these masers. In NGC 4258—currently the most well-defined masing disk—the proposed scenario can also account for the intriguing clustering of the high-velocity maser spots in distinct clumps, the restricted radial distribution of the low-velocity sources, and the dip in the maser spectrum at the systemic velocity of the disk. In this case, we infer a molecular density of ~109 cm-3, a disk mass of ~104 M☉, and a mass accretion rate of order ~7 × 10-3 M☉ yr-1, which is consistent with an advection-dominated accretion flow. These results differ significantly from those of the Neufeld and Maloney model (107.5 cm-3, ~100 M☉, and ~7 × 10-5α M☉ yr-1, respectively). The predicted maser luminosities of the blueshifted and redshifted, high-velocity features in NGC 4258 are consistent with the observations, both in the case of C-type (MHD) shocks and dissociative J-type shocks, where the shock speed is about 20 km s-1. The high-velocity features arise nearly along a diameter through the disk that makes an angle of about 2° with the midline. It does not introduce any noticeable deviation from a Keplerian rotation curve (the velocity gradient across the shock is always perpendicular to the line of sight at a maser location). The corrections to the previously derived black hole mass and galaxy distance are negligible. Predictions include slow systematic drifts in the velocity and position of all the high-velocity features, a systematic displacement in the locations of the high-velocity maser sources from the disk midline, and the existence of circumnuclear disks that are delineated only by high-velocity maser emission.