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

Abstract

We present an echelle spectrum of the Swift GRB 050730, obtained four hours after the burst using the MIKE spectrograph on the Magellan Clay Telescope when the afterglow was at R=17.7. The spectrum reveals a forest of absorption features superimposed on a simple power-law shaped continuum, best described as f_nu(lambda)\propto lambda^{alpha} with alpha =1.88\pm 0.01 over lambda=7000-9000 A. We identify the GRB host at z_GRB=3.96855 based on the hydrogen Lyman absorption series, narrow absorption lines due to heavy ions such as OI, CII, SiII, SII, NiII, FeII, CIV, SiIV, and NV, and fine structure transitions such as OI*, OI**, SiII*, CII*, and FeII*. Together these transitions 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 a neutral hydrogen column density of log N(HI)=22.15\pm 0.05 in the host. (2) The associated metal lines exhibit multiple components over a velocity range of ~80 km/s, with >90% of the neutral gas confined in 20 km/s. (3) Comparisons between different ionic transitions show that the host has little/no dust depletion and has 1/100 solar metallicity. (4) The absorbing gas has much higher density than that of intervening damped Lya absorption (DLA) systems. In addition, we report the identification of an intervening DLA system at z_DLA=3.56439 with log N(HI)=20.3\pm 0.1 and < 5% solar metallicity, a Lyman limit system at z_LLS=3.02209 with log N(HI)=19.9\pm 0.1, a strong MgII absorber at z_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 to reveal a wealth of information in the ISM and the intergalactic medium along the sightline (abridged).