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

Free access article

Issue		A&A Volume 478, Number 2, February I 2008


Page(s)		321 - 333
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20077986

A&A 478, 321-333 (2008)
DOI: 10.1051/0004-6361:20077986

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

B. Borguet, D. Hutsemékers, G. Letawe, Y. Letawe, and P. Magain

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.

Key words: galaxies: quasars: general -- polarization

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