  In popular cosmological scenarios, some time beyond a redshift of 10, stars
within protogalaxies created the first heavy elements; these systems, together
perhaps with an early population of quasars, generated the ultraviolet
radiation and mechanical energy that reheated and reionized the cosmos. The
history of the Universe during and soon after these crucial formative stages is
recorded in the all-pervading intergalactic medium (IGM), which contains most
of the ordinary baryonic material left over from the big bang. Throughout the
epoch of structure formation, the IGM becomes clumpy and acquires peculiar
motions under the influence of gravity, and acts as a source for the gas that
gets accreted, cools, and forms stars within galaxies, and as a sink for the
metal enriched material, energy, and radiation which they eject.
