Constraints on the physical properties of the damped Ly alpha system of Q0000-2619 at z=3.054

We present the detection of C II and C II* absorption in the z = 3.0543 dam ped Ly alpha system toward Q0000-2619. The derived population ratio implies a fine structure excitation temperature between 19.6 and 21.6 K. The upper value sets a strict upper limit on the CMB temperature at this redshift, w hich is consistent with the predicted value of 11.05 K from standard cosmol ogy. Under the assumptions of an ionization degree rang -ing from 0 to 10%, a gas kinetic temperature between 100 and 10 000 K and a UV field with a M ilky Way spectrum, the density of the absorber is constrained to be between 0.7 and 40 cm(-3) and the H-ionizing flux between 1 and 80 times the inten sity of the Galactic UV field. If the damped Ly alpha system is assumed to be homogeneous, the implication is that its size in the direction of the li ne of sight must be between 1 and 100 pc.