Incidence Rate of GRB - HOST - DLAs 3

We study the incidence rate of damped Lyα systems associated with the host galaxies of gamma-ray bursts (GRB-HOST-DLAs) as functions of neutral hydrogen column density (NHI) and projected star formation rate (SFR) using cosmological SPH simulations. Assuming that the occurrence of GRBs is correlated with the local SFR, we find that the median NHI of GRB-HOST-DLAs progressively shifts to lower NHI values with increasing redshift, and the incidence rate of GRB-HOST-DLAs with logNHI > 21.0 decreases rapidly at z ≥ 6. Our results suggest that the likelihood of observing the signature of IGM attenuation in GRB afterglows increases towards higher redshift, because it will not be blocked by the red damping wing of DLAs in the GRB host galaxies. This enhances the prospects of using high-redshift GRBs to probe the reionization history of the Universe. The overall incidence rate of GRB-HOST-DLAs decreases monotonically with increasing redshift, whereas that of QSO-DLAs increases up to z = 6. A measurement of the difference between the two incidence rates would enable an estimation of the value of ηGRB, which is the mass fraction of stars that become GRBs for a given amount of star formation. Subject headings: cosmology: theory — stars: formation — galaxies: evolution – galaxies: formation – methods: numerical