A LOFAR survey of spatially resolved ultra steep spectrum sources

The correlation between radio spectral steepness and redshift has been successfully used to find very high redshift radio galaxies. However, the origin of this correlation is still unknown. Understanding particle acceleration processes is fundamental to explaining the correlation. The ultrasteep spectra of high redshift radio sources means they will be bright at low frequencies. The low frequency regime is also where spectral turnovers or cutoffs will be observed due to different physical mechanisms. Thus the new Low Frequency Array (LOFAR) is an ideal instrument for observing these sources. As part of an ongoing survey, we use international baselines to measure the low-frequency spectra (< 200 MHz) and their spatial distributions along the jets of 11 bright extended steep spectrum high redshift radio sources. We will present early results from our LOFAR low band antenna survey of ultra-steep spectrum radio galaxies, including complementary high band antenna images of one of the targets. From this, we will measure the shape of the integrated spectra at low frequencies and determine whether the spectra change over these spatially resolved sources, thereby constraining particle acceleration processes.These objects are also candidates for searches for carbon radio recombination lines, the detection of which would provide a unique diagnostic for constraining the physical properties of cold gas in high redshift radio galaxies.