2005MNRAS.356.1529H

In order to interpret H₂quasar absorption-line observations of damped Lyα systems (DLAs) and subDLAs, we model their H₂abundance as a function of dust-to-gas ratio, including H₂self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of the gas by using H₂data. Using H₂excitation data for DLAs with H₂detections, we derive a gas density 1.5 ≲ log n/(cm3) ≲ 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 H₂deficient DLAs and subDLAs with H₂fractions 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 H₂fraction (≲10^–6) observed for κ≲ 1/30 (κ is the dust-to-gas ratio in units of the Galactic value); H₂self-shielding causes a rapid increase in, and large variations of, H₂abundance for κ≳ 1/30. We finally propose an independent method to estimate the star formation rates of DLAs from H₂abundances; 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.