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, n_H = 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 z_abs > 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 (n_H= 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.