Modeling Polarization Maps of External Galaxies.

Abstract

We extend previous radiation transfer models of disk galaxies to include bulge plus disk systems. The Monte Carlo radiation transfer models include the polarizing effects of dust scattering and transmission through aligned grains. We assume that the dust grains are aligned by large scale toroidal magnetic fields and we model the interstellar polarization using empirical formulae derived for the Milky Way. Although our galaxy models do not include dust or star inhomogeneities arising from clumping or spiral arm structures, they do successfully reproduce the observed dust lanes and polarization patterns and null points. We apply our techniques to NGC 4565 and NGC 891 which are examples of nearly edge-on spiral galaxies with prominent dust lanes. We can successfully model the polarimetric structure of NGC 4565 with uniformly aligned grains and a smooth dust lane. However, our models fail to reproduce the structure within NGC 891's dust lane. This failure is attributed to the striking disk activity present in NGC 891 and the polarimetric evidence for vertical magnetic field structures. The dust lane activity and non-toroidal magnetic fields in NGC 891 appear to break the large scale grain alignment to such an extent that our smooth model is unable to reproduce the flux and polarization maps. Although NGC 891 is often assumed to be very similar to our own Galaxy, our modeling suggests that, polarimetrically at least, NGC 4565 behaves more like the Milky Way than does NGC 891.