DOPPLER-SHIFT ASYMMETRY IN HIGH-VELOCITY MASER EMISSION FROM SHOCKS IN CIRCUMNUCLEAR DISKS

The rapidly rotating, masing circumnuclear disk in the central subpars ec region of the galaxy NGC 4258 is remarkably circular and Keplerian, yet a striking asymmetry appears in the maser spectrum: the redshifte d, high-velocity sources are much more numerous and significantly more intense than the blueshifted ones. A similar strong asymmetry also ap pears in the recently discovered, masing, circumnuclear disks in NGC 1 068 and NGC 4945, thus suggesting it may be a general phenomenon. We s how that the observed Doppler shift asymmetry can naturally arise due to spiral shocks in circumnuclear disks. We argue that population inve rsion can largely be quenched in these systems because of IR photon tr apping, and that the high-velocity maser emission originates within th in 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 rot ation, the locations of the masers make the blueshifted radiation trav el through a column of noninverted gas that maintains close velocity c oherence with the maser source, where absorption occurs. The resulting asymmetry in the high-velocity maser spectrum, where the redshifted e mission appears systematically stronger, is independent of the existen ce of a warp in the disk or the azimuthal direction to the observer, a nd is insensitive to small distortions in the velocity field in the di sk. The high velocities of these features reflect the rotational veloc ities 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 accou nt for the intriguing clustering of the high-velocity maser spots in d istinct clumps, the restricted radial distribution of the low-velocity sources, and the dip in the maser spectrum at the systemic velocity o f the disk. In this case, we infer a molecular density of similar to 1 0(9) cm(-3), a disk mass of similar to 10(4) M-circle dot, and a mass accretion rate of order similar to 7 x 10(-3) Mo yr-l, which is consis tent with an advection-dominated accretion flow. These results differ significantly from those of the Neufeld and Maloney model (similar or equal to 10(7.5) cm(-3), similar to 100 M-circle dot, and similar to 7 x 10(-5)alpha M-circle dot yr(-1), respectively). The predicted maser luminosities of the blueshifted and redshifted, high-velocity feature s in NGC 4258 are consistent with the observations, both in the case o f 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 al ong a diameter through the disk that makes an angle of about 2 degrees 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 c orrections to the previously derived black hole mass and galaxy distan ce are negligible. Predictions include slow systematic drifts in the v elocity and position of all the high-velocity features, a systematic d isplacement in the locations of the high-velocity maser sources from t he disk midline, and the existence of circumnuclear disks that are del ineated only by high-velocity maser emission.