and Cosmology

6.6 Evolutionary Effects
satellites like ROSAT, some clusters with z ∼ 1.2 have
been found (see Fig. 6.25). The new X-ray satellites
Chandra and XMM-Newton are even more sensitive.
Therefore, one expects them to be able to find clusters
at even higher redshifts; one example for a cluster at z =
1.393 is shown in Fig. 6.46. This example demonstrates
combining deep X-ray images with observations in the
optical and the NIR is an efficient method of compiling
samples of distant clusters.
Through optical methods, it is also possible to identify
galaxy concentrations at very high redshift. One
approach is to assume that luminous AGNs at high
redshift are found preferentially in regions of high overdensity,
which is also expected from models of galaxy
formation. With the redshift of the AGN known, the redshift
at which one should search for an overdensity of
galaxies near the AGN is defined. Those searches have
proven to be quite successful; for instance, they are performed
using narrow-band filter photometry, with the
filter centered on the redshifted Lyα line, tuned to the
redshift of the AGN. Candidates need to be verified
spectroscopically afterwards. One example of a strong
galaxy concentration at z = 4.1 is presented in Fig. 6.47.
The identification of a strong spatial concentration of
galaxies is not sufficient to have identified a cluster
of galaxies though, because it is by no means clear
whether one has found a gravitationally bound system
of galaxies (and the corresponding dark matter).
Rather, such galaxy concentrations are considered to
be the predecessors of galaxy clusters which will only
evolve into bound systems during later cosmological
evolution.
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