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

n order to interpret H2 quasar absorption-line observations of damped Lyα systems (DLAs) and subDLAs, 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 the gas by using H2 data. Using H2 excitation data for DLAs with H2 detections, we derive a gas density 1.5 <~ log n(cm-3) <~ 2.5, temperature 1.5 <~ log T(K) <~ 3, and an internal ultraviolet (UV) radiation field (in units of the Galactic value) 0.5 <~ logχ<~ 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 H2 deficient DLAs and subDLAs 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 κ<~ 1/30 (κ is the dust-to-gas ratio in units of the Galactic value); H2 self-shielding causes a rapid increase in, and large variations of, H2 abundance for κ>~ 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.