Observations of the Gas Reservoir around a Star-Forming Galaxy in the Early Universe

Abstract

We present a high signal-to-noise spectrum of a bright galaxy at z = 4.9 in14 h of integration on VLT FORS2. This galaxy is extremely bright, i_850 =23.10 +/- 0.01, and is strongly-lensed by the foreground massive galaxy clusterAbell 1689 (z=0.18). Stellar continuum is seen longward of the Ly-alphaemission line at ~7100 \AA, while intergalactic H I produces strong absorptionshortward of Ly-alpha. Two transmission spikes at ~6800 Angstroms (A) and ~7040A are also visible, along with other structures at shorter wavelengths.Although fainter than a QSO, the absence of a strong central ultraviolet fluxsource in this star forming galaxy enables a measurement of the H I fluxtransmission in the intergalactic medium (IGM) in the vicinity of a highredshift object. We find that the effective H I optical depth of the IGM isremarkably high within a large 14 Mpc (physical) region surrounding the galaxycompared to that seen towards QSOs at similar redshifts. Evidently, thishigh-redshift galaxy is located in a region of space where the amount of H I ismuch larger than that seen at similar epochs in the diffuse IGM. We argue thatobservations of high-redshift galaxies like this one provide unique insights onthe nascent stages of baryonic large-scale structures that evolve into thefilamentary cosmic web of galaxies and clusters of galaxies observed in thepresent universe.