On the injection spectrum of relativistic electrons in high-redshift
  radio galaxies
  We point out that the remarkable linearity of the ultra-steep radio spectra
of high redshift radio galaxies reflects a previously reported general trend
for powerful radio galaxies, according to which the spectral curvature is
lesser for sources having steeper spectra (measured near rest-frame 1 GHz). We
argue based on existing theoretical and observational evidence that it is
premature to conclude that the particle acceleration mechanism in sources
having straight, ultra-steep radio spectra gives rise to an ultra-steep
injection spectrum of the radiating electrons. In empirical support to this we
show that the estimated injection spectral indices, available for a
representative sample of 35 compact steep spectrum (CSS) radio sources are not
correlated with their rest-frame (intrinsic) rotation measures, which are known
to be typically large, indicating a dense environment, as is also the case for
high-$z$ radio galaxies.
