Galaxy surveys 329Figure 11.6.Evolution of the spectral energy distribution (SED) of a stellar population
modelled as a star formation burst of 3 Gyr, over the lifetime of the universe. From top to
bottom, the SEDs are shown at ages: 0.2, 3.2, 3.4, 4, 5, 10, 18 Gyr. The latest Bruzual and
Charlot spectral synthesis models have been used.
of neutral hydrogen clouds along the sightline (Lyαforest) produces a further
depression blueward of the Lyα, which becomes stronger at higher redshifts. As
a result, a star-forming galaxy atz3 is seen disappearing in the transition from
the B to the U band (‘U drop-out’). In general, by measuring colours, such as
U–B and B–V, one can select a large sample of galaxies aroundz∼3, since
these sources will stand out in a colour–colour diagram, having very red U–B
colours (lyman limit passing through the two filters) and nearly zero B–V colours
(flat spectrum). Such a technique was first successfully applied to ground-based
imaging data (e.g. Steidelet al1996), which have the advantage of covering much
larger solid angles than the HDF, although they cannot match the photometric
accuracy of HST, which is critical to measuring colours accurately.