Molecular hydrogen in damped Lyα systems:: clues to interstellar physics at high redshift

In order to interpret H-2 quasar absorption-line observations of damped Lyalpha systems (DLAs) and subDLAs, we model their H-2 abundance as a function of dust-to-gas ratio, including H-2 self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of the gas by using H-2 data. Using H-2 excitation data for DLAs with H-2 detections, we derive a gas density 1.5 less than or similar to log n(cm(-3)) less than or similar to 2.5, temperature 1.5 less than or similar to log T (K) less than or similar to 3, and an internal ultraviolet (UV) radiation field (in units of the Galactic value) 0.5 less than or similar to log chi 1.5. We then find that the observed relation between the molecular fraction and the dust-to-gas ratio of the sample is naturally explained by the above conditions. However, it is still possible that H-2 deficient DLAs and subDLAs with H-2 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 H-2 fraction (less than or similar to10(6)) observed for kappa less than or similar to 1/30 (kappa is the dust-to-gas ratio in units of the Galactic value); H-2 self-shielding causes a rapid increase in, and large variations of, H-2 abundance for kappa greater than or similar to 1/30. We finally propose an independent method to estimate the star formation rates of DLAs from H-2 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.