Physical conditions in two high-redshift H₂-bearing GRB-DLAs, 120815A and 121024A

The gamma-ray burst (GRB) afterglows provide a unique opportunity to study the interstellar medium (ISM) of star-forming galaxies at high-redshift. The GRB-DLAs (damped Lyman-α absorbers) contain a large neutral hydrogen column density, N(H I), and are observed against the GRB afterglow. A large fraction of GRB-DLAs show presence of molecular hydrogen (H₂) which is an indicator of star-formation. Hence it is important to study those GRB-DLAs which have H₂ lines to decipher and understand their physical conditions. The GRB-DLAs 121024A and 120815A, situated at redshift 2.30 and 2.36, respectively, are two such important H₂-bearing GRB-DLAs. Besides H₂, these two GRB-DLAs also show many metal lines. In this work we have carried out a detail numerical study on the H₂ lines, as well as on those metal lines, in GRB-DLAs 121024A and 120815A self-consistently. We use the spectral synthesis code CLOUDY for this study. This modelling helps us to determine the underlying physical conditions which give rise to such lines and hence to understand these two GRB-DLAs in much more detail than any other previous investigation. We find that the hydrogen densities for these two H₂-bearing DLAs are ≥60 cm⁻³. Moreover our study infers that the linear sizes are ≤17.7 pc for these two GRB-DLAs, and the mean gas temperatures averaged over the cloud thickness, are ≤140 K. Overall, we find that these two H₂-bearing GRB-DLAs are denser, cooler, and smaller compared to those without H₂.