and Cosmology

9.1 Galaxies at High Redshift
the HST (GEMS, a field of 30 ′ × 30 ′ centered on the
CDF-S, and the 2 deg 2 COSMOS survey) will further
improve the statistics of the results obtained from the
HUDF and GOODS.
The multiwavelength approach by GOODS yields an
unprecedented view of the high-redshift Universe. Although
these studies and scientific analyses are ongoing
(at the time of writing), quite a large number of very
high-redshift (z 5) galaxies have already now been
discovered and studied: a sample of more than 500 I-
band drop-outs has been obtained from deep ACS/HST
images. Lyman-break galaxies at z ∼ 6 seem to have
stellar populations with masses and lifetimes comparable
to those at z ∼ 3. This implies that at a time when the
Universe was 1 Gyr old, a stellar population with mass
∼ 3 × 10 10 M ⊙ and age of a few hundred million years
(as indicated by the observed 4000-Angstrom break)
was already in place. This, together with the apparently
high metallicity of these sources, is thus another indication
of how quickly the early Universe has evolved. The
z ∼ 6 galaxies are very compact, with half-light radii of
∼ 1 kpc, and thus differ substantially from the galaxy
population known in the lower-redshift Universe.
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9.1.4 Natural Telescopes
Galaxies at high redshift are faint and therefore difficult
to observe spectroscopically. For this reason, the
brightest galaxies are preferentially selected (for detailed
examination), i.e., basically those which are the
most luminous at a particular z – resulting in undesired,
but hardly avoidable selection effects. For example,
those Lyman-break galaxies at z ∼ 3 for which the red-
Fig. 9.12. A particularly interesting and efficient way to examine
galaxies at high redshift is provided by the strong lensing effect in
clusters of galaxies. Since a gravitational lens can magnify the light
of background galaxies (by magnification of the solid angle), one
can expect to detect apparently brighter galaxies at high redshift
in the background of clusters. Here, an HST image of the cluster
Abell 2390 is shown, in which several lens systems are visible. On
the left, the central region of the cluster is shown. Three systems
with a strong lens effect in this cluster are presented in the blow-ups
at top. In the center, the so-called “straight arc” is visible which has
a redshift of about 0.91. On the right and left, two multiply imaged
systems are displayed, the images indicated by letters; the two
sources associated with these images have redshifts of z = 4.04
and 4.05, respectively