A structure for quasars under the scope of polarization - I. The UV/optical polarization dichotomy of type-1 and type-2 AGN

We present ultraviolet/optical spectropolarimetric modelling of the phenomenologically based structure for quasars proposed by Elvis (2000). In this first paper of a series, we explore the continuum polarization emerging from radiatively accelerated and bent winds that were vertically launched from the accretion disc in an active galactic nucleus (AGN). We simulate the radiative transfer occurring in Thomson scattering and dust extinction media over a range of morphological parameters and optical depths of the wind. We demonstrate that the wind geometry proposed by Elvis with a phenomenologically derived bending angle of θ = 60° still underestimates the observed optical polarization percentage of type-1 and type-2 AGN and does not yet reproduce the expected dichotomy of the polarization position angle. To recover the observed polarization properties, a smaller bending angle and some amount of dust shielding in the equatorial region should be considered. A two-phase outflow is found to generate both the observed polarization dichotomy and acceptable levels of polarization degree if the wind has a bending angle θ = 45°, and the conical shells have a half-opening angle of 3° < δθ < 10°. The absorbing dust column at the wind base should be in the range 1 < τdust ≤ 4 (τ being integrated over 2000-8000 Å). Straightforward observational tests from spectropolarimetry and from determining the number density of different AGN types can be performed to further constrain the wind geometry.