Polarized radiative transfer modeling of warped and clumpy dusty tori

¹ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
e-mail: frederic.marin@astro.unistra.fr
² Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, 3122 Victoria, Australia
³ Universitäts-Sternwarte Muünchen, Scheinerstraße 1, 81679 München, Germany
⁴ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany

Received: 18 May 2017
Accepted: 21 August 2017

Abstract

Context. Active galactic nuclei (AGN) are anisotropic objects surrounded by an optically thick equatorial medium whose true geometry still defies observers.

Aims. We aim to explore the optical scattering-induced polarization that emerges from clumpy and warped dusty tori to check whether they can fit the unified model predictions.

Methods. We ran polarized radiative transfer simulations in a set of warped and non-warped clumpy tori to explore the differences induced by distorted dust distributions. We then included warped tori in a more complex model representative of an AGN to check, using polarimetry and imaging methods, whether warps can reproduce the expected polarization dichotomy between Seyfert-I and Seyfert-II AGN.

Results. The main results from our simulations highlight that isolated warped structures imprint the polarization degree and angle with distinctive signatures at Seyfert-I orientations. Included in an AGN model, the signatures of warps are easily (but not always) washed out by multiple scattering in a clumpy environment. Imaging polarimetry may help to detect warped tori, but we prove that warps can exist in AGN circumnuclear regions without contradicting observations.

Conclusions. Two warped tori with a non-significant difference in geometry in terms of photometry or spectroscopy can have entirely different signatures in polarimetry. Testing the geometry of any alternative model to the usual dusty torus using polarized radiative transfer is a necessary approach to verify or reject a hypothesis.