Molecular hydrogen at high-z: physical conditions in protogalaxies

We study the physical conditions in damped Lyman-α systems (DLAs), using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H2 by Ledoux et al. (2003). H2 is detected in 13-20% of the DLAs in our sample. Using the rotation level populations of H2 and fine-structure excitations of C I we show the mean kinetic temperature of H2 components is 153 ± 78 K, nH = 10-250 cm-3, and the ambient radiation field is similar to or slightly higher than the mean diffuse UV field of the Galaxy. Combining this with the success rate of detecting H2 in DLAs we conclude that at least 13-20% of DLAs at zabs ≥ 1.9 show the presence of CNM and substantial star-formation activity. C II* absorption is detected in all the components where H2 absorption is seen. The level populations of C II in these systems is consistent with the physical parameters derived from the excitation of H2 and C I. We detect C II* in about 50% of the DLAs and therefore in a considerable fraction of DLAs that do not show H2. The absence of C I absorption, the measured N(C II*)/N(C II) and N(Al III)/N(Al II) ratios in these systems are consistent with the gas having lower density (nH = 0.3-6 cm-3) than that seen in the H2 components. 50% of the DLAs that do not show C II* are consistent with them originating from a low density warm neutral medium