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We present deep polarimetric observations at 154 MHz with the MurchisonWidefield Array (MWA), covering 625 deg^2 centered on RA=0 h, Dec=-27 deg. Thesensitivity available in our deep observations allows an in-band,frequency-dependent analysis of polarized structure for the first time at longwavelengths. Our analysis suggests that the polarized structures are dominatedby intrinsic emission but may also have a foreground Faraday screen component.At these wavelengths, the compactness of the MWA baseline distribution providesexcellent snapshot sensitivity to large-scale structure. The observations aresensitive to diffuse polarized emission at ~54' resolution with a sensitivityof 5.9 mJy beam^-1 and compact polarized sources at ~2.4' resolution with asensitivity of 2.3 mJy beam^-1 for a subset (400 deg^2) of this field. Thesensitivity allows the effect of ionospheric Faraday rotation to be spatiallyand temporally measured directly from the diffuse polarized background. Ourobservations reveal large-scale structures (~1 deg - 8 deg in extent) in linearpolarization clearly detectable in ~2 minute snapshots, which would remainundetectable by interferometers with minimum baseline lengths >110 m at 154MHz. The brightness temperature of these structures is on average 4 K inpolarized intensity, peaking at 11 K. Rotation measure synthesis reveals thatthe structures have Faraday depths ranging from -2 rad m^-2 to 10 rad m^-2 witha large fraction peaking at ~+1 rad m^-2. We estimate a distance of 51+/-20 pcto the polarized emission based on measurements of the in-field pulsarJ2330-2005. We detect four extragalactic linearly polarized point sourceswithin the field in our compact source survey. Based on the known polarizedsource population at 1.4 GHz and non-detections at 154 MHz, we estimate anupper limit on the depolarization ratio of 0.08 from 1.4 GHz to 154 MHz.

Low frequency observations of linearly polarized structures in the interstellar medium near the south Galactic pole