The contribution of the unresolved extragalactic radio sources to the diffuse brightness of the sky was evaluated using the source number-flux measurements available in literature. We first optimized the fitting function of the data based on the number counts' distribution. We then computed the brightness temperature at various frequencies from 151 to 8440 MHz and derived its spectral dependence. As expected the frequency dependence can be described by a power law with a spectral index γ ? 2.7, in agreement with the flux emitted by the steep-spectrum sources. The contribution of flat-spectrum sources becomes relevant at frequencies above several GHz. Using the data available in the literature we improved our knowledge of the brightness of the unresolved extragalactic radio sources. The results obtained have general validity and can be used to disentangle the various contributions of the sky brightness and to evaluate the cosmic microwave background (CMB) temperature.
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