Spectral Index - Redshift Relation for Radio Galaxies and Quasars

Cosmological evolution of synchrotron spectra of the powerful extra-galactic radio sources was studied by many authors. Some indications of such an evolution had been found firstly by analysis of ‘spectral index - flux density’ (α — S) relation for the sample of relatively strong radio sources [1, 2]. Later Gopal-Krishna and Steppe [3] extended the analysis to weaker sources and found that the slope of α med (S) curve changes dramatically at intermediate flux densities. Gopal-Krishna and Steppe pointed out that the maxima of the α med (S) curve and of differential source counts are at almost the same flux density ranges (see [3], Fig.2). It has to be noticed that the all mentioned results were obtained using the low-frequency spectral indices and on the basis of low frequency samples.