This paper introduces a new program to find high-redshift radio galaxies in the southern hemisphere through ultra-steep spectrum ( USS ) selection .
We define a sample of 234 USS radio sources with spectral indices \alpha ^ { 843 } _ { 408 } \leq - 1.0 ( S _ { \nu } \propto \nu ^ { \alpha } ) and flux densities S _ { 408 } \geq 200 mJy in a region of 0.35 sr , chosen by cross-correlating the revised 408 MHz Molonglo Reference Catalogue , the 843 MHz Sydney University Molonglo Sky Survey and the 1400 MHz NRAO VLA Sky Survey in the overlap region -40 \degr < \delta < -30 \degr .
We present Australia Telescope Compact Array ( ATCA ) high-resolution 1384 and 2368 MHz radio data for each source , which we use to analyse the morphological , spectral index and polarization properties of our sample .
We find that 85 per cent of the sources have observed-frame spectral energy distributions that are straight over the frequency range 408–2368 MHz , and that , on average , sources with smaller angular sizes have slightly steeper spectral indices and lower fractional linear polarization .
Fractional polarization is anti-correlated with flux density at both 1400 and 2368 MHz .
We also use the ATCA data to determine observed-frame Faraday rotation measures for half of the sample .