A polarimetry study of starburst galaxies

Abstract

Optical imaging polarimetry has been carried out for a number of starburst galaxies under various morphological classifications. In several cases, kpc-scale reflection nebulae are detected which arise from dust grains scattering starburst radiation towards the observer. In general, the scattering medium is displaced up to several kpc from the current star- forming environment and in one case, the dust distribution extends above and below the galactic disk in a manner reminiscent of M82. It is now well established that galactic-scale outflows (superwinds) are prevalent in starburst galaxies and it is tempting to attribute the anomalous dust distributions detected in this thesis with these powerful processes. Furthermore, there is increasing evidence that tidal encounters initiate starburst activity and in some cases a dynamic event of this kind may also be responsible for the observed distribution of scattering material. Mie scattering models were constructed for two of the objects observed in this work. An optically-thin approximation was used which, for many plausible distributions of the dust along the line-of-sight, can be shown to be roughly valid. Although the estimates derived from this technique have a range of values, in all cases the dust component detected in scattered light appears to be at least an order of magnitude more massive than the amount of FIR-emitting dust derived from IRAS data. This is not too surprising - the material detected via polarimetry is probably too cold for IRAS which is sensitive to dust warmer than about 30-50 K.