The Radio Spectra of Sources in the Fourth Cambridge Catalogue-III

Abstract

Using observations at 38, 178, 408, 1410 and 2650 MHz the radio spectra of sources in the declination range |$0^\circ-20^\circ$| of the 4C catalogue have been determined. Over the frequency range 38–1410 MHz simple power-law spectra have been fitted to each source and as a result it has been possible to confirm that over the range of flux densities covered the mean spectral index of sources does not change. It is seen, however, that the mean spectral index of sources selected at any given frequency is a function of that frequency. From observations at 2650 MHz it is clear that the spectra of many sources steepen at frequencies above 1410 MHz, but there is no evidence that this effect is more marked in weaker sources which are assumed to be at greater distances with greater cosmological red-shifts; the steepening is more marked in sources of high brightness temperature. Over the whole range of frequencies covered the distribution of spectra among radio galaxies and quasi-stellar sources are very similar.