An investigation of the redshift-dependence of the radio spectral index among powerful double radio sources

Abstract

Using two flux-limited, meter-wavelength samples of extragalactic sources, for which multifrequency measurements of flux density as well as essentially complete redshift information are now available, the spectral properties are analyzed, based on spectral indices κ_1e and alpha_2e determined at 1 and 2 GHz in the rest frame of the source. Both samples are selected at 408 MHz and represent sources observed at high(≧ 10 Jy) and intermediate (~ 1 Jy) flux densities, for which median values of redshift are about 0.2 and 1.4, respectively. The near constancy of the intrinsic spectral index over the flux density range from ~25 Jy to 1 Jy at 408 MHz, suggests that the steepening of α with z redshift reported in the literature, if real, probably does not continue to early cosmological epochs (z > 1).