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⊙) 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 polarised 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 polarised submm emission is associated with the remnant. No known mechanism would vary the synchrotron polarisation in this way and so we attribute the excess polarised submm flux to cold dust within the remnant, providing fresh evidence that cosmic dust can form rapidly. The inferred dust polarisation fraction is unprecedented (fpol ∼ 30 per cent) which, coupled with the brief timescale available for grain alignment (<300 yr), suggests that supernova dust differs from that seen in other Galactic sources (where fpol = 2 − 7 per cent), or that a highly efficient grain alignment process must operate in the environment of a supernova remnant.