Molecular hydrogen in damped Ly-alpha systems: clues to interstellar   physics at high-redshift
  In order to interpret H2 (molecular hydrogen) quasar absorption line observations of damped Ly-alpha systems (DLAs) and sub-DLAs, we model their H2 abundance as a function of dust-to-gas ratio, including H2 self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of gas by using H2 data. Using H2 excitation data for DLA with H2 detections, we derive a gas density 1.5 < log n [cm^-3] < 2.5, temperature 1.5 < log T [K] < 3, and internal UV radiation field (in units of the Galactic value) 0.5 < log \chi < 1.5. We then find that the observed relation between molecular fraction and dust-to-gas ratio of the sample is naturally explained by the above conditions. However, it is still possible that H2 deficient DLAs and sub-DLAs with H2 fractions less than ~ 10^-6 are in a more diffuse and warmer state. The efficient photodissociation by the internal UV radiation field explains the extremely small H2 fraction (< 10^-6) observed for \kappa < 1/30 (\kappa is the dust-to-gas ratio in units of the Galactic value); H2 self-shielding causes a rapid increase and the large variations of H2 abundance for \kappa > 1/30. We finally propose an independent method to estimate the star formation rates of DLAs from H2 abundances; such rates are then critically compared with those derived from other proposed methods. The implications for the contribution of DLAs to the cosmic star formation history are briefly discussed. 
H. Hirashita, A. Ferrara (SISSA)