Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes
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Abstract
Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within ~10 M_8_ pc of the central 10^8^M_8_ M_sun_ black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). According to the Seyfert unification scheme, Seyfert 2 galaxies represent Seyfert 1 galaxies that are obscured by a dusty torus. Analogous schemes have been proposed for relating narrow-line radio galaxies to broad-line radio galaxies and radio-loud QSOs, and luminous QSOs to ultraluminous infrared galaxies. In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (>~10^5^ K) and hot (>~10^6^ K) gas components that have been inferred to exist on scales <~10^2^ pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Electron scattering by this gas might give rise to the "ionization cones" that have been detected in objects like NGC 1068 (with a corresponding focusing of the X-ray emission possibly contributing to the apparent deficit of X-ray-luminous Seyfert 2 galaxies), and it could also be responsible for some of the continuum optical polarization characteristics of Seyfert galaxies and radio-quiet QSOs. Other observed optical and infrared polarization properties of these AGNs may be attributed to the effects of dust scattering and transmission in the outer zones of the wind. Finally, we discuss the application of this model to the interpretation of the ~0.6 keV X-ray absorption feature reported in several BLOs.