Echelle Spectroscopy of a Gamma-Ray Burst Afterglow at z = 3.969: A New Probe of the Interstellar and Intergalactic Media in the Young Universe

We present an echelle spectrum of the Swift GRB 050730, obtained four hoursafter the burst using the MIKE spectrograph on the Magellan Clay Telescope whenthe afterglow was at R=17.7. The spectrum reveals a forest of absorptionfeatures superimposed on a simple power-law shaped continuum, best described asf_nu(lambda)\propto lambda^{alpha} with alpha =1.88\pm 0.01 overlambda=7000-9000 A. We identify the GRB host at z_GRB=3.96855 based on thehydrogen Lyman absorption series, narrow absorption lines due to heavy ionssuch as OI, CII, SiII, SII, NiII, FeII, CIV, SiIV, and NV, and fine structuretransitions such as OI*, OI**, SiII*, CII*, and FeII*. Together thesetransitions allow us to study the the properties of the interstellar medium(ISM) in the GRB host. The principal results are as follows. (1) We estimate aneutral hydrogen column density of log N(HI)=22.15\pm 0.05 in the host. (2) Theassociated metal lines exhibit multiple components over a velocity range of ~80km/s, with >90% of the neutral gas confined in 20 km/s. (3) Comparisons betweendifferent ionic transitions show that the host has little/no dust depletion andhas 1/100 solar metallicity. (4) The absorbing gas has much higher density thanthat of intervening damped Lya absorption (DLA) systems. In addition, we reportthe identification of an intervening DLA system at z_DLA=3.56439 with logN(HI)=20.3\pm 0.1 and < 5% solar metallicity, a Lyman limit system atz_LLS=3.02209 with log N(HI)=19.9\pm 0.1, a strong MgII absorber atz_MgII=2.25313, and a pair of MgII absorbers at z_MgII=1.7731, 57 km/s apart.We demonstrate that rapid echelle spectroscopy of GRB afterglows helps toreveal a wealth of information in the ISM and the intergalactic medium alongthe sightline (abridged).Comment: 5 pages, including 2 figures; ApJ Letters in press (minor changes in response to the referee's comments