The low-frequency spectra of nonthermal radio sources

Abstract

Flux densities of discrete radio sources measured with the -MHz T-arrays at the Dominion Radio Astrophysical Observatory are combined with observations from higher frequencies to construct and classify the continuum spectra of 225 sources in the range MHz. New or revised flux densities are given for 38 sources at 10 and 22 MHz, together with revised calibration factors for previously published & densities at these frequencies. Scales used for other published flux densities in the range 12-178 MHz are assessed and an internally consistent set of scale revision factors is derived. The spectral properties of complete samples of extragalactic sources selected at 10, 178, and 1400 MHz are c9mpared, using spectral indices evaluated at six frequencies. Classes of sources having unusually high- and low-spectral indices are identified. Quasistellar sources are found to have spectral indices differently from those of radio galaxies; quasistellar sources with power-law regions of their spectra have generally higher spectral indices there than those of the galaxies. A small group of sources identified with blank fields contains some members whose spectral indices are uncharacteristic of distant radio galaxies or of quasistellar sources. There appears to be no evolutionary effect present in the spectral index-luminosity correlation for radia galaxies.