GRB 130606A AS A PROBE OF THE INTERGALACTIC MEDIUM AND THE INTERSTELLAR MEDIUM IN A STAR-FORMING GALAXY IN THE FIRST Gyr AFTER THE BIG BANG

Abstract

We present high signal-to-noise ratio Gemini and MMT spectroscopy of the\noptical afterglow of the gamma-ray burst (GRB) 130606A at redshift z=5.913,\ndiscovered by Swift. This is the first high-redshift GRB afterglow to have\nspectra of comparable quality to those of z~6 quasars. The data exhibit a\nsmooth continuum at near-infrared wavelengths that is sharply cut off blueward\nof 8410 Angs due to absorption from Ly-alpha at redshift z~5.91, with some flux\ntransmitted through the Ly-alpha forest between 7000-7800 Angs. We use column\ndensities inferred from metal absorption lines to constrain the metallicity of\nthe host galaxy between a lower limit of [Si/H]>-1.7 and an upper limit of\n[S/H]<-0.5 set by the non-detection of S II absorption. We demonstrate\nconsistency between the dramatic evolution in the transmission fraction of\nLy-alpha seen in this spectrum over the redshift range z=4.9 to 5.85 with that\npreviously measured from observations of high-redshift quasars. There is an\nextended redshift interval of Delta-z=0.12 in the Ly-alpha forest at z=5.77\nwith no detected transmission, leading to a 3-sigma upper limit on the mean\nLy-alpha transmission fraction of <0.2% (or tau_eff(Ly-alpha) > 6.4). This is\ncomparable to the lowest-redshift Gunn-Peterson troughs found in quasar\nspectra. We set a 2-sigma upper limit of 0.11 on the neutral fraction of the\nIGM at the redshift of the GRB from the lack of a Ly-alpha red damping wing,\nassuming a model with a constant neutral density. Some Ly-beta and Ly-gamma\ntransmission is detected in this redshift window, indicating that it is not\ncompletely opaque, and hence that the IGM is nonetheless mostly ionized at\nthese redshifts. GRB 130606A thus for the first time realizes the promise of\nGRBs as probes of the first galaxies and cosmic reionization.\n