Cassiopeia A: dust factory revealed via submillimetre polarimetry

If Type II supernovae - the evolutionary end points of short-lived, massive stars - produce a significant quantity of dust (> 0.1 M(circle dot)) then they can explain the rest-frame far-infrared emission seen in galaxies and quasars in the first Gyr of the Universe. Submillimetre (submm) observations of the Galactic supernova remnant, Cas A, provided the first observational evidence for the formation of significant quantities of dust in Type II supernovae. In this paper, we present new data which show that the submm emission from Cas A is polarized at a level significantly higher than that of its synchrotron emission. The orientation is consistent with that of the magnetic field in Cas A, implying that the polarized submm emission is associated with the remnant. No known mechanism would vary the synchrotron polarization in this way and so we attribute the excess polarized submm flux to cold dust within the remnant, providing fresh evidence that cosmic dust can form rapidly. This is supported by the presence of both polarized and unpolarized dust emission in the north of the remnant where there is no contamination from foreground molecular clouds. The inferred dust polarization fraction is unprecedented (f(pol) similar to 30 per cent) which, coupled with the brief time-scale available for grain alignment (< 300 yr), suggests that supernova dust differs from that seen in other Galactic sources (where f(pol) = 2-7 per cent) or that a highly efficient grain alignment process must operate in the environment of a supernova remnant.