and Cosmology

9. The Universe at High Redshift
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energy, and the tidal component of the gravitational field
removes stars, gas, and dark matter away from it; the
Magellanic Stream (see Fig. 6.6) is presumably the result
of such a process. Only after several orbits – the
number of which depends on the initial conditions and
on the mass ratio – will the satellite galaxy finally merge
with the larger one.
Results from Semi-Analytic Models. The free parameters
in semi-analytic models – such as the star-formation
efficiency or the fraction of energy from SNe that
is transferred into the gas – are fixed by comparison
with some key observational results. For example, one
requires that the models reproduce the correct normalization
of the Tully–Fisher relation and that the number
counts of galaxies match those observed. Although
these models are too simplistic to trace the processes
of galaxy evolution in detail, they are highly successful
in describing the basic aspects of the galaxy population,
and they are continually being refined. For instance, this
model predicts that galaxies in clusters basically consist
of old stellar populations, because here the merger
processes were already concluded quite early in cosmic
history. Therefore, at later times gas was no longer
available for the formation of stars. Figure 9.38 shows
the outcome of such a model in which the merger history
of the individual halos has been taken straight from
the numerical N-body simulation, hence the spatial locations
of the individual galaxies are also described by
these simulations.
By comparison of the results from such semi-analytic
models with the observed properties of galaxies and
their spatial distribution, the models can be increasingly
refined. In this way, we obtain more realistic
descriptions of those processes which are included in
the models in a parametrized form. This comparison is
of central importance for achieving further progress in
our understanding of the complex processes that are
occurring in galaxy evolution, which can neither be
studied in detail by observation, nor be described by
more fundamental simulations.
As a result of such models, the correlation function
of galaxies as it is obtained from the Millennium
simulation (see Sect. 7.5.3) is presented in Fig. 9.39,
in comparison to the correlation function observed
in the 2dFGRS. The agreement between the model
and the observations is quite impressive; both show
a nearly perfect power law. In particular, the correlation
function of galaxies distinctly deviates from the
correlation function of dark matter on small scales,
implying a scale-dependent bias factor. The question
arises as to which processes in the evolution of galaxies
may produce such a perfect power law: why does
the bias factor behave just such that ξ g attains this simple
shape. The answer is found by analyzing luminous
and less luminous galaxies separately, or galaxies with
and without active star formation – for each of these
subpopulations of galaxies, ξ g is not apowerlaw.For
this reason, the simple shape of the correlation function
shown in Fig. 9.39 is probably a mere coincidence
(“cosmic conspiracy”).
Another result from such models is presented in
Fig. 9.40, also from the Millennium simulation (see
Sect. 7.5.3). Here, one of the most massive dark matter
halos in the simulation box at redshift z = 6.2 is
shown, together with the mass distribution in this spatial
region at redshift z = 0. In both cases, besides the distribution
of dark matter, the galaxy distribution is also
Fig. 9.39. Correlation function of galaxies at z = 0 (filled
circles connected by the solid curve), computed from the Millennium
simulation in combination with semi-analytic models
of galaxy evolution. This is compared to the observed galaxy
correlation function as derived from the 2dFGRS (diamonds
with error bars). The dashed curve shows the correlation
function of matter