Cosmological observations in the radio domain: the contribution of extragalactic sources

The low frequency tail of the CMB spectrum, down along the radio range (~1 GHz), may carry weak spectral distortions which are fingerprints of processes occurred during different epochs of the thermal history of the Universe, from z ~ 3 × 106 to reionization. TRIS and ARCADE2 are the most recent experiments dedicated to the exploration of this chapter of CMB cosmology. The level of instrumental accuracy they reached in the determination of the absolute sky temperature is such that the removal of galactic and extra-galactic contamination is the true bottleneck towards the recovery of the cosmological signal. This will be certainly the case also for future experiments in the radio domain. Here we present an update of a study originally done to recognize the contribution of unresolved extra-galactic radio sources to the sky brightness measured by TRIS. Despite the specific context which originated our analysis, this is a study of general interest, improved by the inclusion of all the source counts available up-to-date from 150 MHz to 8.4 GHz.