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. RM grids serve as highly sensitive tools for probing magnetic fields in various astrophysical environments, offering an ensemble of point-like probes that illuminate foreground magneto-ionic structures, thus enabling detailed studies of cosmic magnetism. 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.
The figure above shows a comparison of NVSS (left) and POSSUM (right) RMs within the Fornax cluster, showcasing POSSUM’s transformative capability to probe magnetised gas in clusters, groups, and many other degree-scale extragalactic objects. The background is a Digitized Sky Survey optical image (greyscale). Solid circles indicate the position, magnitude, and sign of RMs measured by each survey, with diameters proportional to (RM)2 and colours indicating RM sign (red for positive, blue for negative).