  High-velocity galactic outflows, driven by intense bursts of star formation
and black hole accretion, are invoked by current theories of galaxy formation
to terminate star formation in the most massive galaxies and to deposit heavy
elements in the intergalactic medium. From existing observational evidence on
high-redshift galaxies, it is unclear whether such outflows are localized to
regions of intense star formation just a few kiloparsecs in extent, or whether
they instead have a significant impact on the entire galaxy and its
surroundings. Here we present two-dimensional spectroscopy of a star-forming
galaxy at redshift z=3.09 (seen 11.5 Gyr ago, when the Universe was 20 per cent
of its current age): its spatially extended Ly-alpha emission appears to be
absorbed by HI in a foreground screen covering the entire galaxy, with a
lateral extent of at least 100 kpc and remarkable velocity coherence. It was
plausibly ejected from the galaxy during a starburst several 1E8 yr earlier and
has subsequently swept up gas from the surrounding intergalactic medium and
cooled. This demonstrates the galaxy-wide impact of high-redshift superwinds.
