Returning to Figure 1, it must be
remembered that everything we have learnt on the distant
universe up to this point required `decoding' the information `encrypted'
in the absorption spectra of QSOs. It is easy to appreciate, therefore, the
strong incentive which motivated astronomers
in the 1990s to detect high resdshift galaxies directly.
After many years of fruitless searches, we have witnessed since 1995 a
veritable explosion of data from the
Hubble Deep Fields and ground-based surveys
with large telescopes. The turning point
was the realisation of the effectiveness
of the Lyman break technique (Figure 19) in
preselecting candidate
z 
3 galaxies
(Steidel et al. 1996).
Although these galaxies constitute only ~ 3 - 4%
of the thousands of faint objects, at all redshifts,
revealed by a moderately deep CCD exposure at the prime focus of a 4m
telescope (see Figure 20),
they can be readily distinguished on the basis
of their colours alone, when observed through appropriately
selected filters (Figure 21).
 |
Figure 19. (Courtesy of Kurt Adelberger).
An illustration of the principles behind the
Lyman break technique. Hot stars have flat
far-UV continua, but emit fewer photons
below 912Å, the limit of the Lyman series
of hydrogen (top panel). These photons are also efficiently
absorbed by any H I associated with the sites
of star formation (middle panel) and have a short mean free
path - typically only ~ 40Å - in the IGM at z = 3.
Consequently, when observed from Earth (bottom panel),
the spectrum of a star forming galaxy at
z |
 |
Figure 20. A typical deep CCD image
recorded at the prime focus of a 4m-class telescope. This particular
image (of the field designated DSF2237b)
was obtained with the COSMIC camera
of the Palomar Hale telescope, by exposing for a total
of two hours through a custom made
|
 |
Figure 21. All the ~ 3300 galaxies from Figure 20 are included in this colour-colour plot. The shaded region shows how the 140 candidate Lyman break galaxies are selected for subsequent spectroscopic follow-up. The symbol size is proportional to the object magnitude; circles denote objects detected in all three bands, while triangles are lower limits in (Un - G) for Un dropouts. |
The Lyman break technique has been very successful
at finding high redshift galaxies thanks to the combination
of the increasingly large and UV sensitive CCDs used
to identify candidates on the one hand,
and the multi-object spectroscopic capabilities of
large telescopes required for follow-up and
confirmation on the other. Thus, samples of spectroscopically confirmed
z 3 galaxies
have grown from zero to more than one thousand in the
space of only five years. As can be seen from
Figure 22,
the spectroscopic redshifts generally conform to
expectations based on just two colours. Such large samples
have made it possible to trace the star formation history of the
universe over most of the Hubble time and to measure the large-scale
properties of this population of galaxies, most notably their clustering and
luminosity functions (see
Steidel 2000
for a review).
 |
Figure 22. The location of
spectroscopically confirmed
galaxies (from the surveys by Steidel and collaborators)
on the (Un - G) vs. (G -
|
In parallel with this work on the Lyman break population as a whole, in the last few years we have also begun to study in more detail the physical properties of some of the brighter galaxies in the sample. The questions which we would like to address are:
Many of these questions link observational cosmology with stellar and interstellar astrophysics, and this will become evident as we now explore them in turn.
)
and red in (Un - G). For this reason, such
galaxies are sometimes referred to as U-dropouts.
A more quantitative description of the Lyman break technique
can be found in