Probing the Interstellar Medium near Star‐forming Regions with Gamma‐Ray Burst Afterglow Spectroscopy: Gas, Metals, and Dust

We study the chemical abundances of the interstellar medium surrounding highz gamma-ray bursts (GRBs) through analysis of the damped Lya systems (DLAs)identified in afterglow spectra. These GRB-DLAs are characterized by large HIcolumn densities N(HI) and metallicities [M/H] spanning 1/100 to nearly solar,with median [M/H]>-1. The majority of GRB-DLAs have [M/H] values exceeding thecosmic mean metallicity of atomic gas at z>2, i.e. if anything, the GRB-DLAsare biased to larger metallicity. We also observe (i) large [Zn/Fe] values(>+0.6) and sub-solar Ti/Fe ratios which imply substantial differentialdepletion, (ii) large a/Fe ratios suggesting nucleosynthetic enrichment bymassive stars, and (iii) low C^0/C^+ ratios (<10^{-4}). Quantitatively, theobserved depletion levels and C^0/C^+ ratios of the gas are not characteristicof cold, dense HI clouds in the Galactic ISM. We argue that the GRB-DLAsrepresent the ISM near the GRB but not gas directly local to the GRB (e.g. itsmolecular cloud or circumstellar material). We compare these observations withDLAs intervening background quasars (QSO-DLAs). The GRB-DLAs exhibit largerN(HI) values, higher a/Fe and Zn/Fe ratios, and have higher metallicity thanthe QSO-DLAs. We argue that the differences primarily result fromgalactocentric radius-dependent differences in the ISM: GRB-DLAs preferentiallyprobe denser, more depleted, higher metallicity gaslocated in the inner few kpcwhereas QSO-DLAs are more likely to intersect the less dense, less enriched,outer regions of the galaxy. Finally, we investigate whether dust obscurationmay exclude GRB-DLA sightlines from QSO-DLA samples; we find that the majorityof GRB-DLAs would be recovered which implies little observational bias againstlarge N(HI) systems.Comment: 16 pages, 9 figures. Submitted to Ap