Constraints on the physical properties of the damped Ly α system of Q 0000 − 2619 at z = 3 . 054

We present the detection of C II and C II * absorption in the z = 3.0543 damped Lyα 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 10 000 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 Lyα 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.