Molecular hydrogen and the nature of damped Lyman-alpha systems

We report results from our mini-survey for molecular hydrogen in eight high -redshift damped Lyman-cr (DLA) systems using the ESO Ultra-violet and Visi ble Spectrograph on the VLT. Ln addition, we investigate two systems using ESO public data. We include in the sample the only system where H-2 was pre viously detected and studied at high-spectral resolution. Altogether our sa mple consists of eleven absorbers with 1.85 < z(abs) < 3.4. We confirm the presence of H-2 in the z(abs) = 2.3377, metal-poor ([Si/H] = -1.20), system toward PKS 1232+082. The derived molecular fraction, f = 2N (H-2)/(2N(H-2)+N(H I)) = 4x10-4, is two orders of magnitude less than what has been claimed previously from low-resolution data. The physical conditio ns within the cloud can be constrained directly from observation. The kinet ic temperature and particle density are in the ranges, respectively, 100 < T < 300 K and 30 < n(H) < 50 cm(-3) In addition, UV pumping is of the same order of magnitude than in our Galaxy The upper limits on the molecular fraction derived in nine of the systems a re in the range 1.2 x 10(-7) - 1.6 x 10(-5). There is no evidence in this s ample for any correlation between H-2 abundance and relative heavy element depletion into dust grains. This should be investigated using a larger samp le however. The molecular abundance in a few DLA systems land in particular in the two systems where H-2 is detected) is consistent with what is seen in the Magellanic clouds. But most of the DLA measurements are well below t hese values. This is probably partly due to small amounts of dust and/or hi gh UV flux. We argue however that the lack of molecules is a direct consequ ence of high kinetic temperature (T > 3000 K) implying a low formation rate of H-2 onto dust grains. Therefore, most of the DLA systems; arise in warm and diffuse neutral gas.