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

We present UV/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 polarisation 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 theta = 60deg still underestimates the observed optical polarisation percentage of type-1 and type-2 AGN and does not yet reproduce the expected dichotomy of the polarisation position angle. To recover the observed polarisation 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 polarisation dichotomy and acceptable levels of polarisation degree if the wind has a bending angle theta = 45deg, and the conical shells have a half-openingangle of 3deg < delta_theta < 10deg. The absorbing dust column at the wind base should be in the range of 1 < tau_dust < 4 (tau being integrated over 2000 - 8000 Angs). Straightforward observational tests from spectropolarimetry and from determining the number density of different AGN types can be performed to further constrain the wind geometry.