On the injection spectrum of relativistic electrons in high-redshift radio galaxies

TL;DR

This paper examines the spectral properties of high-redshift radio galaxies, challenging the assumption that their ultra-steep spectra indicate an ultra-steep electron injection spectrum, and finds no correlation with environmental factors.

Contribution

It questions the link between ultra-steep spectra and electron injection spectra in high-redshift radio galaxies, based on observational data and theoretical considerations.

Findings

No correlation between injection spectral indices and rotation measures.

Spectral curvature is less for sources with steeper spectra.

High-redshift radio galaxies do not necessarily have ultra-steep injection spectra.

Abstract

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.