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 number counts distribution. We then computed the brightness temperature at various frequencies from 151 MHz 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 literature we improved our knowledge of the brightness of the unresolved extragalactic radio sources. The results obtained have general validity and they can be used to disentangle the various contributions of the sky brightness and to evaluate the CMB temperature.

At frequencies close to 1 GHz the sky diffuse radiation is a superposition of radiation of Galactic origin, the 3 K relic or cosmic microwave background radiation, and the signal produced by unresolved extragalactic sources. Because of their different origin and space distribution, the relative importance of the three components varies with frequency and depends on the direction of observation. With the aim of disentangling the components we built TRIS, a system of three radiometers, and studied the temperature of the sky at ¼ 0:6, 0.82, and 2.5 GHz using geometrically scaled antennas with identical beams (HPBW ¼ 18; 23 ). Observations included drift scans along a circle at constant declination ¼ þ42, which provided the dependence of the sky signal on the right ascension, and absolute measurement of the sky temperature at selected points along the same scan circle. TRIS was installed at Campo Imperatore (latitude ¼ 42 26 0 north, longitude ¼ 13 33 0 , elevation ¼ 2000 m a.s.l.) in central Italy, close to the Gran Sasso Laboratory.

With the TRIS experiment we have performed absolute measurements of the sky brightness in a sky circle at ¼ þ42at the frequencies ¼ 0:60, 0.82, and 2.5 GHz. In this paper we discuss the techniques used to separate the different contributions to the sky emission and give an evaluation of the absolute temperature of the CMB. For the blackbody temperature of the CMB we get T (1 ), while the second one is systematic. These results represent a significant improvement with respect to the previous measurements. We have also set new limits to the free-free distortions, À6:3 ; 10 À6 < Y A < 12:6 ; 10 À6 , and slightly improved the Bose-Einstein upper limit, jj < 6 ; 10 À5 , both at 95% confidence level.