Is there an unaccounted excess Extragalactic Cosmic Radio Background?

Abstract

Analyses of measurements of the distribution of absolute brightness temperature over the radio sky have led recently to suggestions that there exists a substantial unexplained extragalactic radio background. Consequently, there have been numerous attempts to place constraints on plausible origins for the `excess'. We suggest here this expectation of a large extragalactic background, over and above that contributed by the sources observed in the surveys, is based on an extremely simple geometry adopted to model the Galactic emission and the procedure adopted in the estimation of the extragalactic contribution. In this paper, we derive the extragalactic radio background from wide-field radio images by using a more realistic modeling of the Galactic emission and decompose the sky maps at 150, 408, and 1420 MHz into anisotropic Galactic and isotropic extragalactic components. The anisotropic Galactic component is assumed to arise from a highly flattened spheroid representing the thick disc, embedded in a spherical halo, both centered at the Galactic center, along with Galactic sources, filamentary structures, Galactic loops and spurs. All components are constrained to be positive and the optimization scheme minimizes the sky area occupied by the complex filaments. We show that in contrast to the simple modeling of the Galactic emission as a plane parallel slab, the more realistic modeling yields estimates for the uniform extragalactic brightness that is consistent with expectations from known extragalactic radio source populations.