dropouts
Although PG searches over the last 20 years have proved
disappointing in uncovering a distant population of monolithic PGs, it is
precisely this lack of success which has given rise in recent years to
renewed interest in galaxy evolution at more modest
redshift (z 
1-3), where
galaxies are detected in numerous
quantities by, for example, the Hubble Space Telescope.
These studies of evolution have proved pivital in our understanding the evolution of local galaxies, not by way of uncovering PGs as such - this is one discovery which has alluded the Hubble Telescope so far, but by measuring the rate at which stars are formed in normal spiral galaxies over a look-back time of a few Gyrs and have produced a rapid advance in studies of the Universe in the era of the 10m-class telescopes.
The basic technique used to find galaxies at these redshifts
involves broad-band optical imaging in a number of filters to detect the
sudden drop in the spectral energy distribution shortward of the
redshifted Lyman break (
900(1 +
z) Å). Photons with shorter
wavelengths than this ionize hydrogen and are re-processed primarily into Lyman
photons. The resultant
characteristic spectral ``step'' can be seen
in Figure 3 for a zero redshift
galaxy. In a
series of experiments starting in 1992, Steidel and collaborators (see
Steidel et
al. 1996)
have used this technique along with follow-up
deep spectroscopy on the Keck telescope, to confirm the existence of a
substantial population of compact (~ 0.4 arcsec) star-forming
galaxies at z ~ 3. From the strength of their emission lines and
shape of
their continua, these galaxies have SFRs 30 M
yr-1 and a number density equivalent to between 50%-10%
of the space density
of present day bright galaxies. Progress in this area has been very rapid and
it is now possible for the first time to sketch out the star-forming
history of a substantial fraction of the present day galaxy population
(Madau et
al. 1996):
The work of Steidel, together with other
samples selected in a similar manner - including
galaxies found by the Hubble Space Telescope (e.g.
Lilly et al.
1996,
Connolly et
al. 1997),
indicate that the overall SFR of galaxies
increases from z = 0 to z = 2, during which time a
significant fraction
of the heavy elements in the Universe are formed, and then tails off
towards higher redshift - see Figure 4 taken
from
Madau et al.
(1998).
Although the uncertainties are considerable and many surveys are still
underway, Figure 4 is a real landmark in
cosmology as it represents the very
first attempt to map out the star-forming history of galaxies.
|
| Figure 4. The implied star-formation rate (SFR) in
M yr-1 against
redshift inferred from several optically selected galaxy samples. The
plot is taken from
Madau et
al. 1998.
|
The Lyman dropout galaxies are known to be young from the shape of
their spectral energy distributions, which are quite flat (see
Figure 3),
and they also show signs of energetic outflows consistent with intense
bursts of star formation
(Pettini et
al. 1998).
So in key respects this
widespread population of objects at z ~ 3 has some of the essential
characteristics which we expect from PGs and it is quite
possible that we are seeing directly, for the first time, the formation
of present day galaxies. These results certainly demonstrate beyond any
reasonable doubt that massive galaxy formation was well underway by
z = 3.5. This relatively late epoch of galaxy formation appears to be in
general
agreement with the popular models of galaxy formation in which the mass
density
of the Universe is dominated by cold dark matter, discussed in
section 3,
which predict a peak SFR of a few M yr-1 between
2 z 4
(Baugh et
al. 1998).