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

We present the detection of CII and CII* absorption in the z = 3.0543 damped 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, which is consistent with the predicted value of 11.05 K from standard cosmology. Under the assumptions of an ionization degree ranging from 0 to 10%, a gas kinetic temperature between 100 and 10000 K and a UV field with a Milky 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 intensity 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 line of sight must be between 1 and 100 pc.