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 4 hr 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 superposed on a simple power-law shaped continuum, best described as fν(λ) ∝ λα with α = 1.88 ± 0.01 over λ = 7000-9000 Å. We identify the gamma-ray burst (GRB) host at zGRB = 3.96855 based on the hydrogen Lyman absorption series, narrow absorption lines due to heavy ions such as O I, C II, Si II, S II, Ni II, Fe II, C IV, Si IV, and N V, and fine-structure transitions such as O I*, O I**, Si II*, C II*, and Fe II*. Together these transitions allow us to study 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(H ) = 22.15 ± 0.05 in the host. (2) The associated metal lines exhibit multiple components over a velocity range of ~80 km s-1, with >90% of the neutral gas confined in 20 km s-1. (3) Comparisons between different ionic transitions show that the host has little or no dust depletion and has 1/100 solar metallicity. (4) The absorbing gas has much higher density than that of intervening damped Lyα absorption (DLA) systems. In addition, we report the identification of an intervening DLA system at zDLA = 3.56439 with log N(H ) = 20.3 ± 0.1 and <5% solar metallicity, a Lyman limit system at zLLS = 3.02209 with log N(H ) = 19.9 ± 0.1, a strong Mg II absorber at z = 2.25313, and a pair of Mg II absorbers at z = 1.7731, 57 km s-1 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 sight line, which, when followed up with late-time deep imaging, will allow us to uncover a sample of distant galaxies with known ISM properties to constrain galaxy formation models.

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