The cosmic microwave background radiation temperature at z(abs)=3.025 toward QSO 0347-3819

From the analysis of the C+ fine-structure population ratio in the damped Lyalpha system at z(abs) = 3.025 toward the quasar Q0347-3819 we derive an upper bound of 14.6 +/- 0.2 K on the cosmic microwave background temperature (T-CMBR) regardless the presence of other different excitation mechanisms. The analysis of the ground state rotational level populations of H-2 detected in the system reveals a Galactic-type UV radiation field ruling out UV pumping as an important excitation mechanism for C+. The low dust content estimated from the Cr/Zn ratio indicates that the IR dust emission can also be neglected. When the collisional excitation is considered, we measure a temperature for the cosmic background radiation of T-CMBR = 12.1(-3.2)(+1.7) K. The results are in agreement with the T-CMBR = 10.968 +/- 0.004 K predicted by the hot Big Bang cosmology at z(abs) = 3.025.