Modeling optical and UV polarization of AGNs V. Dilution by interstellar
  polarization and the host galaxy
  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. 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. We upgrade 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 include in
our analysis tool a routine that may add, depending on the user's objectives,
an interstellar component. Using a generic AGN model, we illustrate how
interstellar polarization and starlight dilution impact the observed
polarimetric signal of AGN. We apply 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 derive
several intrinsic parameters such as the system inclination and the torus
opening angle. 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.
