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