Dissecting the active galactic nucleus in Circinus -- III. VLT/FORS2 polarimetry confirms dusty cone illuminated by a tilted accretion disc

We present polarimetric maps of the Circinus galaxy nucleus in the $BVRI$
bands, obtained with VLT/FORS2. Circinus is the closest Seyfert 2 galaxy and
harbours an archetypal obscured active galactic nucleus (AGN). Recent high
angular resolution imaging revealed that a major fraction of its mid-infrared
(MIR) emission is coming from the polar region. Previously, we demonstrated
that these observations are consistent with a model of a compact dusty disc and
a hyperboloid shell, resembling a hollow cone on larger scales. Here we focus
on the AGN core, up to 40 pc from the central engine, and compare the
observations to the radiative transfer models. Polarization maps reveal a
conical structure, coinciding with the ionization cone. The
wavelength-dependence of the polarization degree indicates that scattering on
dust grains is producing polarization. The observed polarization degree
($\approx1-3\%$) is lower than predicted by the models; however, this is only a
lower limit, since stellar emission dominates the total flux in the optical.
The observed polarization angle ($\approx30$ degrees) is reproduced by the
model of a dusty disc with a hollow cone that is illuminated by a tilted
anisotropic central source. An accretion disc aligned with the ionization cone
axis, and alternative dust geometries, such as a paraboloid shell, or a torus
enveloped by ambient dust, are inconsistent with the data. We conclude that the
optical polarimetric imaging supports earlier evidence for the presence of dust
in the polar region, tentatively associated with dusty outflows.