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 \approx 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 .