# S-PASS/ATCA: a window on the magnetic universe in the southern hemisphere

## TL;DR

The S-PASS/ATCA survey provides the first wide-band radio polarimetry map of the southern sky, revealing that Galactic foregrounds dominate RM and most sources have simple Faraday structures, with implications for RM measurements.

## Contribution

This work introduces the first wide-band polarimetric survey of southern sky compact sources, demonstrating the dominance of Galactic foregrounds and the simplicity of Faraday structures.

## Key findings

- Galactic foregrounds dominate RM in the survey area.
- Most sources have a single RM, confirming previous narrow-band results.
- Spectral index differences can lead to errors in polarization analysis.

## Abstract

We present S-PASS/ATCA, the first wide-band radio polarimetry survey of compact sources in the southern sky. We describe how we selected targets for observations with the Australia Telescope Compact Array (ATCA) in the 16 cm band (1.3 - 3.1 GHz), our observing and calibration strategy, how we analysed the data, and how we tested the quality of the data. The data are made publicly available. The survey contains on average one source per five square degrees and has an angular resolution at 2.2 GHz of ~ 2'x1'. Sources with |RM|s > 150 rad m-2 are seen towards the Galactic plane and bright Hii regions, but are rare elsewhere on the sky. Sightlines that are separated by up to 3' show very similar RMs. Based on this observation, we argue that the Galactic foreground is the dominant contributor to RM, confirming previous results, and that the sources must have very simple distributions of Faraday-rotating and synchrotron-emitting media. Many sources that emit at a single RM have a spectral index in linear polarization that is (very) different from the spectral index in Stokes I. Analysing ratios of flux densities Q/I and U/I (to correct for spectral index effects) then leads to erroneous results. About 80 per cent of sightlines in our survey are dominated by emission at only one RM. Therefore, RMs that were determined previously from narrow-band observations at these frequencies are still safe to use.