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

TL;DR

This study uses VLT/FORS2 polarimetry to confirm a dusty cone illuminated by a tilted accretion disc in the Circinus galaxy's active nucleus, supporting models of dust in polar outflows and the ionization cone.

Contribution

First polarimetric maps of Circinus nucleus in multiple bands confirming the dusty cone illuminated by a tilted accretion disc, aligning with radiative transfer models and previous infrared findings.

Findings

Polarization maps reveal a conical structure matching the ionization cone.

Wavelength dependence indicates dust scattering causes polarization.

Model of a tilted accretion disc with a hollow cone explains observed polarization angle.

Abstract

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.