Observing the First Stars, One Star at a Time

Gamma-Ray Bursts (GRBs) are believed to originate in compact remnants (black holes or neutron stars) of massive stars. Their high luminosities make them detectable out to the edge of the visible universe [23, 12]. GRBs offer the opportunity to detect the most distant (and hence earliest) population of massive stars, one star at a time. In the hierarchical assembly process of halos which are dominated by cold dark matter, the first galaxies should have had lower masses (and lower stellar luminosities) than their low-redshift counterparts. Consequently, the characteristic luminosity of galaxies or quasars is expected to decline with increasing redshift. GRB afterglows, which already produce a peak flux comparable to that of quasars or starburst galaxies at z ∼ 1 − 2, are therefore expected to outshine any competing source at the highest redshifts, when the first dwarf galaxies have formed in the universe. Preliminary polarization data from the Wilkinson Microwave Background Probe (WMAP) indicates an optical depth to electron scattering of ∼ 17±4% after cosmological recombination [27]. This implies that the first stars must have formed at a redshift z ∼ 20 [33, 11, 13, 28] and reionized a substantial fraction of the intergalactic hydrogen around that time. Early reionization can be achieved with plausible star formation parameters in the standard ΛCDM cosmology; in fact, the required optical depth can be achieved in a variety of very different ionization histories (since WMAP places only an integral constraint on these histories [19]). One would like to probe the full history of reionization in order to disentangle the properties and formation history of the stars that are responsible for it. GRB afterglows offer the opportunity to detect stars as well as to probe the ionization state [5] and metal enrichment level [16] of the intervening intergalactic medium (IGM).