Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...452..565k&link_type=abstract
Astrophysical Journal v.452, p.565
Astronomy and Astrophysics
Astronomy
46
Galaxies: Active, Galaxies: Nuclei, Galaxies: Seyfert, Polarization, Radiative Transfer, Galaxies: Quasars: General, Scattering
Scientific paper
I have computed the 1200-8000 A thermal continuum polarization induced by gas and dust arranged in configurations compatible with current active galactic nuclei (AGNs) unification schemes. Both uniform- density tori and stratified-density disk-driven winds were considered. A Monte Carlo radiative transfer code was developed which includes the polarization mechanisms of electron and dust scattering as well as dichroic extinction by aligned grains. A Galactic-type grain population was assumed. Based on these calculations, I propose a new interpretation of many of the observed polarization traits of Seyfert galaxies and QSOs: namely, that the polarization in these sources is induced by the same optically thick material which is assumed to obscure the central engine in unification schemes.
In particular, I suggest that stratified-density winds could provide a natural explanation (and one consistent with unification models) of the polarization trends observed in Seyfert galaxies. Such winds can display polarizations (P ≲ 20%) oriented perpendicular to the axis along viewing angles inclined to the axis by θ0 ≳ 45° in well-collimated winds, this polarization shifts to smaller magnitudes (P ≲ 2%) and parallel orientations for more face-on viewing, consistent with the patterns observed in Seyfert 2 and Seyfert 1 sources, respectively. In less-collimated winds, scattering alone tends to produce parallel orientations for all viewing angles; perpendicular polarization at large θ0 can result if there is a high degree of magnetic grain alignment. The simplest torus models (i.e., uniform-density, opaque gas and dust) do not reproduce this flip in polarization position angle. Furthermore, they generally display high polarization magnitudes (P ≳ 10%) along most viewing angles θ0 > θ∞ (where θ is the torus half-opening angle) and negligible polarization along θ0 > θ∞.
Unlike previous models for AGN polarization which invoke scattering by optically thin electron or dust clouds and generally predict very large polarization (P ≲ 50%) and a single orientation of the polarization position angle, optically thick gas and dust generate smaller polarization magnitudes (P ≲ 30% for tori and P ≲ 20% for winds) with a variety of possible position angles. The magnitude of P depends on the opening angle and optical depth of the torus or wind, the degree of grain alignment, and the location of the continuum source within the obscuring material. The polarization position angle is defined by the primary photon scattering planes within the gas and dust and the direction of grain alignment. It may be oriented either perpendicular or parallel to the symmetry axis, or along an intermediate angle determined by the grain alignment. Several polarization wavelength dependencies are possible. Over the UBVRI wavelength bands, P(λ) may rise into the blue, rise into the red, or remain roughly constant depending on the torus/wind optical depth, degree of grain alignment, and viewing angle. Between 1200 Å and 3600 Å, however, P(λ) rises steadily toward shorter wavelengths for all models, as a result of the increase of polarizations induced per scattering by dust grains in the ultraviolet. Thus, UV spectropolarimetry could provide a powerful test for the presence of dust scattering in AGNs.
Finally, I consider the collimation of the central continuum emission by the obscuring dust and gas. It is shown that disk-driven winds, as a result of their strong density stratification, are effective collimators which can produce apparent cones of ionizing photons even though they lack the sharp edges of uniform-density tori.
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