Stellar disc - Active nucleus alignments in the SDSS

We study the orientations of type I and II AGN galaxies in the Sloan Digital Sky Survey Data Release 4 (SDSS DR4), via the distribution of projected axis ratios of AGN hosts. We define control samples of non-AGN galaxies that mimic the colour, luminosity and concentration distributions of the AGN population, taking into account the effects of dust extinction and reddening. We find that the structural parameters of the control samples are consistent with the full galaxy population of the SDSS DR6, and do not strongly depend on the Seyfert type of the parent AGN sample. By assuming that AGN populations have the same underlying shapes as their corresponding control samples we find that the type I AGN population is biased toward face-on galaxies, while the spiral type II AGN toward edge-on galaxies. Elliptical type II galaxies show a much weaker tendency, consistent with random orientations. These tendencies can be explained by the presence of a central obscuring torus of ≈ 40 degrees of average azimuthal height, parallel to the galactic plane. These findings allow us to rule-out random orientations of AGN torus with respect to their host with a confidence of δ_χ² ∼ 10 for spirals and δ_χ² ≈ 1 for ellipticals.