and Cosmology

9. The Universe at High Redshift
368
shift is verified spectroscopically are among the most
luminous of their kind. The sensitivity of our telescopes
is insufficient in most cases to spectroscopically analyze
a rather more typical galaxy at z ∼ 3.
The magnification by gravitational lenses can substantially
alter the apparent magnitude of sources;
gravitational lenses can then act as natural (and inexpensive!)
telescopes. Examples are the arcs in clusters
of galaxies: many of them have a very high redshift, are
magnified by a factor 5, and hence are brighter by
about ∼ 1.5 mag than they would be without the lens
effect (see Fig. 9.12). It should be mentioned that a factor
of 5 in magnification corresponds to a factor 25 in
the exposure time required for spectroscopy. 1
An extreme example of this effect is represented
by the galaxy cB58 at z = 2.72, which is displayed in
Fig. 9.13. It was discovered in the background of a gal-
1 This factor of 25 makes the difference between an observation that is
feasible and one that is not. Whereas the proposal for a spectroscopic
observation of 3 hours exposure time at an 8-m telescope may be
successful, a similar proposal of 75 hours would be hopelessly doomed
to failure.
Fig. 9.13. The image on the left was taken by the Hubble Space
Telescope. It shows the cluster of galaxies MS1512+36, which
has a redshift of z = 0.37. To the right, and slightly above the
central cluster galaxy, an extended and apparently very blue
object is seen, marked by an arrow. This source is not physically
associated with the cluster but is a background galaxy
at a redshift of z = 2.72. With this HST image it was proved
that this galaxy is strongly lensed by the cluster and, by means
of this, magnified by a factor of ∼ 30. Due to the magnification,
this Lyman-break galaxy is the brightest normal galaxy
at redshift z ∼ 3, a fact that can be profitably used for a detailed
spectroscopic analysis. On the right, a small section
from a high-resolution VLT spectrum of this galaxy is shown.
The Lyα transition of the galaxy is located at λ = 4530 Å,
visible as a broad absorption line. Absorption lines at shorter
wavelengths originate from the Lyα-forest along the line-ofsight
(indicated by short vertical lines) or by metal lines from
the galaxy itself (indicated by arrows)
axy cluster and is magnified by a factor ∼ 30. Hence,
it appears brighter by more than three magnitudes than
a typical Lyman-break galaxy. For this reason, the most
detailed spectra of all galaxies at z ∼ 3 have been taken
of this particular source.
One can argue that there is a high probability
that the flux of the apparently most luminous sources
from a particular source population is magnified by
lensing. The apparently most luminous IRAS galaxy,
F10214+47, is magnified by a factor ∼ 50 by the
lens effect of a foreground galaxy (where the exact
value of the magnification depends on the wavelength,
since the intrinsic structure and size of the source is
wavelength-dependent – hence the magnification is differential).
Other examples are the QSOs B1422+231
and APM 08279+5255, which are among the brightest
quasars despite their high redshifts. In both cases, multiple
images of the QSOs were discovered, verifying
the action of the lens effect. Their magnification, and
therefore their brightness, renders these sources preferred
objects for QSO absorption line spectroscopy
(see Fig. 5.40). The Lyman-break galaxy cB58 men-