The origin of the extragalactic background radiation between 0.5 and 400 MHz

Abstract

The calculations described in this paper show that the superposition of extragalactic radio sources forms a large fraction of the extragalactic radio background spectrum between 0.5 and 400 MHz derived from observations. A significant contribution at low frequencies is also expected from the low-luminosity sources which are inadequately represented by the complete sample used. Synchrotron self-absorption in the components of the individual sources which make up the background results in a low-frequency turn-over. This result is independent of the form of cosmological evolution used, and the spectra for both evolving and non-evolving models peak in the range 1–2 MHz. No other absorption effects are important in this frequency range in the sources which give rise to most of the extragalactic background radiation. The form of the spectrum is due to the fact that for non-evolving models the dominant sources at all frequencies are the low-intensity sources which also have synchrotron turn-overs at low frequencies. For evolving models the contributions from the most luminous sources are sufficiently redshifted for the peak to occur at approximately the same frequency. Most of the flux originates at moderate redshifts for all models, so the possibility of absorption by galaxies or intergalactic gas resulting in a further cut-off is very small.