The Polarisation Sky Survey of the Universe’s Magnetism (POSSUM) is an ASKAP survey that will create a comprehensive Faraday rotation measure (RM) grid of up to one million compact extragalactic sources in the southern sky. The power of RM grids is primarily determined by their sky coverage, areal density, precision, and reliability of the RM measurements. To date, the NRAO VLA Sky Survey (NVSS) RM catalogue remains by far the largest single contribution to the set of known RMs. Gaensler et al. review the POSSUM science case and give an overview of POSSUM’s observations, data processing, outputs, and its complementarity with other radio and multi-wavelength surveys. POSSUM will enable the discovery and detailed investigation of magnetised phenomena in a wide range of cosmic environments, including the intergalactic medium and cosmic web, galaxy clusters and groups, active galactic nuclei and radio galaxies, the Magellanic System and other nearby galaxies, galaxy halos and the circumgalactic medium, and the magnetic structure of the Milky Way across a very wide range of scales, as well as the interplay between these components.

The image above shows a comparison of NVSS and ASKAP EMU/POSSUM maps in a typical extragalactic field. The top left panel shows the NVSS Stokes I (total intensity) map, while the top right panel presents the corresponding NVSS linearly polarised intensity. The bottom left panel displays the ASKAP Stokes I map from the EMU survey, and the bottom right panel shows the linearly polarised intensity derived from RM synthesis using POSSUM data. Notably, POSSUM reveals approximately 27 polarised detections in this region, whereas the NVSS data shows no catalogued polarised sources. The POSSUM polarisation map also reveals faint ISM emission structure, even in a largely empty field, a feature frequently observed in POSSUM data but not seen in NVSS.