Evidence of a type 1/type 2 dichotomy in the correlation between quasar optical polarization and host-galaxy/extended emission position angles


Institut d'Astrophysique et de Géophysique, University of Liège, Allée du 6 Août 17, 4000 Liège, Belgium e-mail: b.borguet@ulg.ac.be

Received: 31 May 2007
Accepted: 16 October 2007

Abstract

Aims.For Seyfert galaxies, the AGN unification model provides a simple and well-established explanation of the type 1/type 2 dichotomy through orientation-based effects. The generalization of this unification model to the higher luminosity AGNs that quasars are remains a key question. The recent detection of type 2 radio-quiet quasars seems to support such an extension. We propose a further test of this scenario.

Methods.On the basis of a compilation of quasar host-galaxy position angles consisting of previously published data and of new measurements performed using HST Archive images, we investigate the possible existence of a correlation between the linear polarization position angle and the host-galaxy/extended emission position angle of quasars.

Results.We find that the orientation of the rest-frame UV/blue extended emission is correlated to the direction of the quasar polarization. For type 1 quasars, the polarization is aligned with the extended UV/blue emission, while these two quantities are perpendicular in type 2 objects. This result is independent of the quasar radio loudness. We interpret this (anti-)alignment effect in terms of scattering in a two-component polar+equatorial model that applies to both type 1 and type 2 objects. Moreover, the orientation of the polarization –and then of the UV/blue scattered light– does not appear correlated to the major axis of the stellar component of the host-galaxy measured from near-IR images.