The optical polarization signatures of fragmented equatorial dusty structures in Active Galactic Nuclei

TL;DR

This study compares the polarization signatures of toroidal and flared disk models of AGN circumnuclear regions, revealing distinct polarization patterns that can help determine their true geometry.

Contribution

It demonstrates that polarization signatures differ significantly between toroidal and flared disk models, providing a new method to distinguish AGN structures.

Findings

Polarization signatures differ markedly between the two geometries.

Varying torus size affects polarization, while flared disk polarization remains stable.

Clumpy flared disks can reach polarization degrees of about 10% with variable angles.

Abstract

If the existence of an obscuring circumnuclear region around the innermost regions of active galactic nuclei (AGN) has been observationally proven, its geometry remains highly uncertain. The morphology usually adopted for this region is a toroidal structure, but other alternatives, such as flared disks, can be a good representative of equatorial outflows. Those two geometries usually provide very similar spectroscopic signatures, even when they are modeled under the assumption of fragmentation. In this lecture note, we show that the resulting polarization signatures of the two models, either a torus or a flared disk, are quite different from each other. We use a radiative transfer code that computes the 2000 - 8000 angstrom polarization of the two morphologies in a clumpy environment, and show that varying the sizes of a toroidal region has deep impacts onto the resulting polarization, while the polarization of flared disks is independent of the outer radius. Clumpy flared disks also produce higher polarization degrees (about 10 % at best) together with highly variable polarization position angles.