2002A&A...381L..64M

From the analysis of the C⁺ fine-structure population ratio in the damped Lyα system at z_abs=3.025 toward the quasar Q0347-3819 we derive an upper bound of 14.6±0.2K 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 H₂detected 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^+1.7_–3.2K. The results are in agreement with the T_CMBR=10.968±0.004K predicted by the hot Big Bang cosmology at z_abs=3.025.