Recent high-resolution imaging observations of water masers in the Seyfert 2 galaxies NGC 4258 and NGC 1068 demonstrate that the masing gas is located within a thin annulus undergoing Keplerian rotation. In both objects, the inner radius of the annulus is located at a significant distance ( ~ 0.1 pc and ~ 1 pc, respectively) from the center of rotation. The masers appear to be confined close to the annular midplane (to within ~ 15(deg) latitutde in NGC 1068 and ~ 1(deg) in NGC 4258). Previous studies of the maser emission have considered molecular gas in alpha -disk models. The location of the inner radius of the masing annulus was assumed to coincide with an invoked disk warp, so that X-rays from the AGN continuum source could penetrate the gas and induce water production. In the case of NGC 4258, self-consistent calculations were made to identify the requisite disk parameters most conducive to the onset of masing conditions. Such disks, however, appear to require nearly ideal mass-to-energy conversion efficiencies to reconcile the observed radiated luminosity with the low inferred accretion rates. We propose an alternative scenario in which the maser flux arises from dense clouds embedded in a magnetized accretion disk wind. Gas and dust lifted by the wind from the disk surface provides an effective radiation shield from the central continuum source, inducing optimal conditions for maser emission within the clouds. In this scheme, the location of the inner edge of the masing annulus is closely related to the dust sublimation radius, where significant radiation shielding commences. Because of the highly stratified density in the wind, shielding is greatest near the disk surface (where the velocity profiles are nearly Keplerian) and rapidly declines at higher latitudes, confining the masing gas to an effective disk-like configuration. Our disk wind model is consistent with observational evidence that the central engines of both NGC 1068 and NGC 4258 are obscured from our direct view and eject high-velocity outflows from their cores.
American Astronomical Society Meeting Abstracts #188
- Pub Date:
- May 1996