Modeling optical and UV polarization of AGNs

V. Dilution by interstellar polarization and the host galaxy

Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
e-mail: frederic.marin@astro.unistra.fr

Received: 13 April 2018
Accepted: 23 May 2018

Abstract

Context. One of the main challenges for polarimetric observations of active galactic nuclei (AGN) is to properly estimate the amount of parasitic light that contaminates the polarization signal. Removing this unpolarized flux is a complex task that has only been achieved in a couple of objects.

Aims. In this fifth paper of the series, we present a new version of the Monte Carlo code STOKES that accounts for dilution by interstellar polarization and host starlight in radiative transfer modeling.

Methods. We upgraded our code by including spectral energy distribution (SED) templates for a set of representative host galaxies. The unpolarized light emitted by those hosts alters the observer polarization while being coherently radiatively coupled to the AGN structure. We also included in our analysis tool a routine that may add, depending on the user’s objectives, an interstellar component.

Results. Using a generic AGN model, we illustrate how interstellar polarization and starlight dilution impact the observed polarimetric signal of AGN. We applied our code to NGC 1068, an archetypal edge-on AGN and demonstrate that STOKES can reproduce its SED, the expected wavelength-dependent polarimetric signatures, and the observed high-angular resolution polarimetric maps. Using the flexibility of the code, we derived several intrinsic parameters such as the system inclination and the torus opening angle.

Conclusions. The new version of our publicly available code now allows observers to better prepare their observations, interpret their data and simulate the three-dimensional geometry and physics of AGN in order to probe unresolved structures. Additionally, the radiative interaction between the host and the AGN can be used to probe the co-evolution of the system.