Observations of the gas reservoir around a star-forming galaxy in the early universe

We present a high signal-to-noise spectrum of a bright galaxy at z=4.9 in 14 hr of integration on VLT FORS2. This galaxy is extremely bright, i ₈₅₀ p 23.10 ± 0.01, and is strongly lensed by the foreground massive galaxy cluster A1689 (z=0.18). Stellar continuum is seen longward of the Lyα emission line at-7100 å, while intergalactic H i produces strong absorption shortward of Lyα. Two transmission spikes at-6800 and-7040 å are also visible, along with other structures at shorter wavelengths. Although this star-forming is galaxy fainter than a QSO, the absence of a strong central ultraviolet flux source in it enables a measurement of the H i flux transmission in the intergalactic medium (IGM) in the vicinity of a high-redshift object. We find that the effective H i optical depth of the IGM is remarkably high within a large 14 Mpc (physical) region surrounding the galaxy compared to that seen toward QSOs at similar redshifts. Evidently, this high-redshift galaxy is located in a region of space where the amount of H i is much larger than that seen at similar epochs in the diffuse IGM. We argue that observations of high-redshift galaxies like this one provide unique insights into the nascent stages of baryonic large-scale structures that evolve into the filamentary cosmic web of galaxies and clusters of galaxies observed in the current universe.