Low frequency observations of linearly polarized structures in the
  interstellar medium near the south Galactic pole
  We present deep polarimetric observations at 154 MHz with the Murchison
Widefield Array (MWA), covering 625 deg^2 centered on RA=0 h, Dec=-27 deg. The
sensitivity available in our deep observations allows an in-band,
frequency-dependent analysis of polarized structure for the first time at long
wavelengths. Our analysis suggests that the polarized structures are dominated
by intrinsic emission but may also have a foreground Faraday screen component.
At these wavelengths, the compactness of the MWA baseline distribution provides
excellent snapshot sensitivity to large-scale structure. The observations are
sensitive to diffuse polarized emission at ~54' resolution with a sensitivity
of 5.9 mJy beam^-1 and compact polarized sources at ~2.4' resolution with a
sensitivity of 2.3 mJy beam^-1 for a subset (400 deg^2) of this field. The
sensitivity allows the effect of ionospheric Faraday rotation to be spatially
and temporally measured directly from the diffuse polarized background. Our
observations reveal large-scale structures (~1 deg - 8 deg in extent) in linear
polarization clearly detectable in ~2 minute snapshots, which would remain
undetectable by interferometers with minimum baseline lengths >110 m at 154
MHz. The brightness temperature of these structures is on average 4 K in
polarized intensity, peaking at 11 K. Rotation measure synthesis reveals that
the structures have Faraday depths ranging from -2 rad m^-2 to 10 rad m^-2 with
a large fraction peaking at ~+1 rad m^-2. We estimate a distance of 51+/-20 pc
to the polarized emission based on measurements of the in-field pulsar
J2330-2005. We detect four extragalactic linearly polarized point sources
within the field in our compact source survey. Based on the known polarized
source population at 1.4 GHz and non-detections at 154 MHz, we estimate an
upper limit on the depolarization ratio of 0.08 from 1.4 GHz to 154 MHz.
