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30th of October 2023
by Thomson et al.
The wide area, high angular resolution, and broad bandwidth provided by the low-band Rapid ASKAP Continuum Survey (RACS-low) has allowed the production of a next-generation rotation measure (RM) grid across the entire Southern Sky. Thomson et al. introduce this project as Spectral and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). In their first data release, they image 30 RACS-low fields in Stokes I, Q, U at 25 arcsecond angular resolution, across 744 to 1032 MHz with 1 MHz spectral resolution. The images are corrected for ionospheric Faraday rotation, primary beam attenuation, and on- and off-axis leakage. The result is a broadband polarised radio component catalogue that contains 5818 RM measurements over an area of ∼1300 square degrees: this areal density of ~4 RMs per square degree is an increase of ∼4 times over the previous state-of-the-art. As a result, despite having used just 3% of the RACS-low sky area, the team have produced the 3rd largest RM catalogue to date. This catalogue has broad applications for studying astrophysical magnetic fields; notably revealing remarkable structure in the Galactic RM sky.

The selection of 30 representative RACS-low fields for analysis in linear polarisation was based on several factors. First, a contiguous subset of the 704 non-Galactic fields was chosen at an intermediate declination, allowing for comparison with previous large-area surveys. A region of interest within the Galaxy with a large angular extent was selected: fields towards the Spica Nebula, a Galactic H II region ionised by the nearby star Spica (Alpha Virginis). The image above shows the sky coverage of SPICE-RACS DR1. The tiling of fields for the entirety of RACS-low is shown in light green, and the 30 fields selected for this data release in dark green. In the inset panel the Stokes I rms noise in the region surrounding the Spica nebula is shown. In white contours, the emission from the nebula itself is shown in Hα from the Wisconsin H-Alpha Mapper (WHAM), with the position of the star Spica shown with a white star.

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