Gamma-Ray Bursts as a Probe of the Very High Redshift Universe

Abstract

The recent evidence that the light curves and the spectra of the afterglows of GRB 970228 and GRB 980326 appear to contain a SN component, in addition to a relativistic shock wave component, provide direct clues that at least the long GRBs detected by BeppoSAX are a result of the collapse of massive stars. We show that, if this is indeed the case, GRBs and their afterglows provide a powerful probe of the very high redshift (VHR) (z > 5) universe. We first establish that GRBs and their afterglows are both detectable out to VHRs. We then show that one expects GRBs to occur out to at least z ~ 10 and possibly z \~ 15 - 20, redshifts that are far larger than those expected for the most distant quasars. This implies that there are large numbers of GRBs with peak photon number fluxes below the detection thresholds of BATSE and HETE-2, and even below the detection threshold of Swift. The mere detection of VHR GRBs would give us our first information about the earliest generations of stars. We show that GRBs and their afterglows can be used as beacons to locate core collapse SNe at redshifts z >> 1, and to study the properties of these SNe. We describe the expected properties of the absorption-line systems and the Ly-alpha forest in the spectra of GRB afterglows, and discuss various strategies for determining the redshifts of VHR GRBs. We then show how the absorption-line systems and the Ly-alpha forest visible in the spectra of GRB afterglows can be used to trace the evolution of metallicity in the universe, and to probe the large-scale structure of the universe at VHRs. Finally, we show how measurement of the Ly-alpha break in the spectra of GRB afterglows can be used to constrain, or possibly measure, the epoch at which re-ionization of the universe occurred, using the Gunn-Peterson test.