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We present a high signal-to-noise spectrum of a bright galaxy at z = 4.9 in 14 h of integration on VLT FORS2. This galaxy is extremely bright, i850 = 23:10  0:01, and is strongly-lensed by the foreground massive galaxy cluster Abell 1689 (z = 0:18). Stellar continuum is seen longward of the Ly emission line at  7100 A, while intergalactic H I produces strong absorption shortward of Ly . Two transmission spikes at 6800 A and 7040 A are also visible, along with other structures at shorter wavelengths. Although fainter than a QSO, the absence of a strong central ultraviolet ux source in this star forming galaxy enables a measurement of the H I ux 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 towards 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 on the nascent stages of baryonic large-scale structures that evolve into the filamentary cosmic web of galaxies and clusters of galaxies observed in the present universe.


This paper was harvested from and ArXiv identifier is arXiv:0808.0921v2