  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, i_850 =
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-alpha
emission line at ~7100 \AA, while intergalactic H I produces strong absorption
shortward of Ly-alpha. Two transmission spikes at ~6800 Angstroms (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 flux
source in this star forming galaxy 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 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.
