2016MNRAS.459.3234S

We present results of our numerical simulation of three H₂-bearing damped Lyman α absorbers (DLAs) in the redshift interval ∼ 2-3. The systems we have modelled are the DLAs at z_abs=2.3377 towards the quasar LBQS 1232 + 0815, at z_abs=2.418 37 towards SDSS J143912.04 + 111740.5 and at z_abs=2.6265 towards FBQS J081240.6 + 320808. We have used the spectral synthesis code cloudy to simulate the physical environment of these DLAs, and constrain the density, radiation field, geometry and dust-grain properties of the DLAs self-consistently based on the observed column densities of various atomic and molecular species such as H i, fine structure lines of C i and the rotational level population of H₂. In our models, we explore the effect of grain size distribution on the predicted column densities of different species. Within the allowed uncertainties in the inferred dust-to-gas ratio, both models with standard interstellar medium grains and smaller sized grains reproduce the observations equally well. Improved constraints on dust-to-gas ratio and line-of-sight extinction are important for probing the grain size distribution in high-z DLAs. We find the H₂-bearing clouds to have line-of-sight thickness in the range 1-6 pc, consistent with what has been found using partial coverage or 21-cm observations in some high-z DLAs.