Spectrum of the Extra-galactic Background Radiation at Low Radio Frequencies

Abstract

THE cosmic noise background radiation observed at metre and decametre wavelengths is generally considered to result from several different spatial components. These include synchrotron radiation from cosmic ray electrons in the galactic disk, similar radiation from the galactic halo (although the relative importance of this component is a matter of considerable controversy), and an isotropic radiation component from the integrated emission of all unresolved extra-galactic radio sources. Results of recent analyses of metre-wave measurements^1–3 place the brightness I_x of the isotropic component at about one-third the minimum total brightness observed at a frequency f of 100 MHz. The spectral index α_x of the isotropic radiation is thought to be about equal to the average spectral index (∼0.8) observed for surveys of extra-galactic discrete sources⁴ at f ∼ 100 MHz where