and Cosmology

9. The Universe at High Redshift
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nearly completely hidden to the optical eye because of
strong dust absorption.
In this chapter, we will attempt to provide an impression
of astronomy of the distant Universe, and shed
light on some interesting aspects that are of particular
importance for our understanding of the evolution of
the Universe. This field of research is currently developing
very rapidly, so we will simply address some of the
main topics in this field today. We begin in Sect. 9.1 with
a discussion of methods to specifically search for highredshift
galaxies, and we will then focus on a method by
which galaxy redshifts can be determined solely from
photometric information in several bands (thus, from the
color of these objects). This method can be applied to
deep sky images observed by HST, and we will present
some of the results of these HST surveys. Finally, we
will emphasize the importance of gravitational lenses as
“natural telescopes”, which, due to their magnification,
provide us with a deeper view into the Universe.
Gaining access to new wavelength domains paves
the way for the discovery of new kinds of sources;
in Sect. 9.2 we will present galaxy populations which
have been identified by submillimeter and NIR observations,
and whose relation to the other known types
of galaxies is yet to be uncovered. In Sect. 9.3 we will
show that, besides the CMB, background radiation also
exists at other wavelengths, but whose nature is considerably
different from that of the CMB. The question of
when and by what processes the Universe was reionized
will be discussed in Sect. 9.4. Then, in Sect. 9.5,
we will focus on the history of cosmic star formation,
and show that at redshift z 1 the Universe was much
more active than it is today – in fact, most of the stars
which are observed in the Universe today were already
formed in the first half of cosmic history. This empirical
discovery is one of the aspects that one attempts
to explain in the framework of models of galaxy formation
and evolution. In Sect. 9.6 we will highlight
some aspects of these models and their link to observations.
Finally, we will discuss the sources of gamma-ray
bursts. These are explosive events which, for a very
short time, appear brighter than all other sources of
gamma rays on the sky put together. For about 25 years
the nature of these sources was totally unknown; even
their distance estimates were spread over at least seven
orders of magnitude. Only since 1997 has it been
known that these sources are of extragalactic origin.
9.1 Galaxies at High Redshift
In this section we will first consider the question of how
distant galaxies can be found, and how to identify them
as such. The properties of these high-redshift galaxies
can then be compared with those of galaxies in the
local Universe, which were described in Chap. 3. The
question then arises as to whether galaxies at high z,and
thus in the early Universe, look like local galaxies, or
whether their properties are completely different. One
might, for instance, expect that the mass and luminosity
of galaxies are evolving with redshift. Examining the
galaxy population as a function of redshift, one can trace
the history of global cosmic star formation and analyze
when most of the stars visible today have formed, and
how the density of galaxies changes as a function of
redshift. We will investigate some of these questions in
this and the following sections.
9.1.1 Lyman-Break Galaxies (LBGs)
How to Find High-Redshift Galaxies? Until about
1995 only a few galaxies with z > 1 had been known;
most of them were radio galaxies discovered by optical
identification of radio sources. The most distant
normal galaxy with z > 2 then was the source of the
giant luminous arc in the galaxy cluster Cl 2244−02
(see Fig. 6.31). Very distant galaxies are faint, and so
the question arises of how galaxies at high z can be
detected at all.
The most obvious answer to this question may perhaps
be by spectroscopy of a sample of faint galaxies.
This method is not feasible though, since galaxies with
R 22 have redshifts z 0.5, and spectra of galaxies
with R > 22 are only observable with 4-m telescopes
and with a very large investment of observing time.
Also, the problem of finding a needle in a haystack
arises: most galaxies with R 24.5 have redshifts z 2
(a fact that was not known before 1995), so how can
we detect the small fraction of galaxies with larger
redshifts?
Narrow-Band Photometry. A more systematic method
that has been applied is narrow-band photometry. Since
hydrogen is the most abundant element in the Universe,