Polarization as a probe of thick dust disk in edge-on galaxies: application to NGC 891

TL;DR

This study uses radiative transfer models to show that a thick dust disk and magnetic field effects are essential to explain observed polarization in edge-on galaxy NGC 891.

Contribution

The paper introduces a radiative transfer model including a thick dust layer and magnetic field effects to explain polarization in edge-on galaxies.

Findings

A thick dust layer is necessary to reproduce polarization structure.

Polarization levels match observed 1-4% outside dust lanes.

Clumpiness and magnetic fields enhance polarization predictions.

Abstract

Radiative transfer models were developed to understand the optical polarizations in edge-on galaxies, which are observed to occur even outside the geometrically thin dust disk, with a scale height of ~ 0.2 kpc. In order to reproduce the vertically extended polarization structure, we find it is essential to include a geometrically thick dust layer in the radiative transfer model, in addition to the commonly-known thin dust layer. The models include polarizations due to both dust scattering and dichroic extinction which is responsible for the observed interstellar polarization in the Milky Way. We also find that the polarization level is enhanced if the clumpiness of the interstellar medium, and the dichroic extinction by vertical magnetic fields in the outer regions of the dust lane are included in the radiative transfer model. The predicted degree of polarization outside the dust lane was found to be consistent with that (ranging from 1% to 4%) observed in NGC 891.