The POlarised GLEAM Survey (POGS) I: First Results from a Low-Frequency
  Radio Linear Polarisation Survey of the Southern Sky
  The low-frequency polarisation properties of radio sources are poorly
studied, particularly in statistical samples. However, the new generation of
low-frequency telescopes, such as the Murchison Widefield Array (MWA; the
precursor for the low-frequency component of the Square Kilometre Array) offers
an opportunity to probe the physics of radio sources at very low radio
frequencies. In this paper, we present a catalogue of linearly-polarised
sources detected at 216 MHz, using data from the Galactic and Extragalactic
All-sky MWA (GLEAM) survey. Our catalogue covers the Declination range
$-17^{\circ}$ to $-37^{\circ}$ and 24 hours in Right Ascension, at a resolution
of around 3 arcminutes. We detect 81 sources (including both a known pulsar and
new pulsar candidate) with linearly-polarised flux densities in excess of 18
mJy across a survey area of approximately 6400 square degrees, corresponding to
a surface density of 1 source per 79 square degrees. The level of Faraday
rotation measured for our sources is broadly consistent with those recovered at
higher frequencies, with typically more than an order of magnitude improvement
in the uncertainty compared to higher-frequency measurements. However, our
catalogue is likely incomplete at low Faraday rotation measures, due to our
practice of excluding sources in the region where instrumental leakage appears.
The majority of sources exhibit significant depolarisation compared to higher
frequencies; however, a small sub-sample repolarise at 216 MHz. We also discuss
the polarisation properties of four nearby, large-angular-scale radio galaxies,
with a particular focus on the giant radio galaxy ESO 422$-$G028, in order to
explain the striking differences in polarised morphology between 216 MHz and
1.4 GHz.
