Thomson et al. present the second data release (DR2) of Spectra and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). SPICE-RACS DR2 is derived from the third low-band epoch of the Rapid ASKAP Continuum Survey (RACS-low3) and covers the entire sky from the South celestial pole up to a declination of +49 degrees; approximately 87.5 % of the celestial sphere. The team produce ‘cutout’ spectral cubes in Stokes I, Q, U around 4 million radio sources and extract spectra towards 5 million radio components. The polarisation catalogue contains the detection of 2.5 × 105 Faraday rotation measures (RM). This places SPICE-RACS DR2 as the largest single RM catalogue ever produced by nearly an order of magnitude; the number of RMs in our catalogue alone is ∼5 times larger than every previous RM catalogue combined.
The resulting RM grid has an areal density of 7 per square degree, providing an effective ‘resolution’ of ∼23 arcminutes, and reveals striking features across the sky. The image above shows rotation measures across the survey area in Galactic coordinates using nearest-neighbour interpolation using diverging colour map and a linear scale to highlight different features of the RM sky. Many of the familiar large-scale features of the Faraday sky emerge from this image. On smaller scales, our improved RM density has now resolved a plethora of Galactic features. Of particular note are striking filamentary RM structures, many of which appear to be very linear as projected on the sky. The dataset also reveals features that extend completely across both the Northern and Southern Galactic poles, which resemble similar structures seen in neutral interstellar medium tracers such as HI or interstellar dust.
One unique large-scale feature in the Southern RM sky is G353-34. This large ring-like structure was first discovered in 2008 in diffuse polarisation images at 1.4 GHz as a depolarisation feature. In the RM grid G353-34 appears as a large ring of enhanced positive RM. The precise nature of the object has not been determined, however it is assumed to be a nearby supernova remnant based on its morphology. The breadth and quality of the SPICE-RACS DR2 dataset will enable a new generation of RM science. All data products are publicly available on the CSIRO Data Access Portal (DAP) and the CSIRO ASKAP Science Data Archive (CASDA).