Polarimetry of the peculiar elliptical galaxy, NGC 5128

Abstract

A high resolution linear polarization map is presented of the peculiar elliptical galaxy NGC5128 (= Centaurus A) and is used to investigate the polarizing mechanism at work in the dust lane. The spatial distribution and orientation of the polarization, together with a possible correlation between the degree of polarization and the extinction through the dust, suggest scattering from aligned non-spherical grains as the probable cause of the polarization. A computer model of the passage of light from an extended source, through a dust cloud of arbitrary shape, has been developed, using a backward Monte-Carlo method. This model has been applied to an NGC5128-like system and it is found that in order to obtain the structure and degree of polarization observed in NGC5128, the dust needs to be aligned by a toroidal magnetic field of about the same strength as that in our Galaxy. It is suggested that the magnetic field could have originally belonged to a spiral galaxy which subsequently collided with NGC5128.