and Cosmology

9.6 Galaxy Formation and Evolution
distribution. Therefore, the planar satellite distribution
has been considered as a further potential problem for
CDM-like models. However, using semi-analytic models
of galaxy formation, combined with simulations of
the large-scale structure, a different picture emerges.
Since galaxies preferentially form in filaments of the
large-scale structure, the accretion of smaller mass halos
onto a high-mass halo occurs predominantly in the
direction of the filament. The most massive sub-halos
therefore tend to form a planar distribution, not unlike
the one seen in the Milky Way’s satellite distribution.
The anisotropy of the distribution of massive satellites
may also serve to explain the Holmberg effect.
Major Mergers. In the framework of semi-analytic
models, a spheroidal stellar population may form in
a “major merger”, which may be defined in terms of the
mass ratio of the merging halos (e.g., larger than 1:3)
– the disk populations of the two merging galaxies are
dynamically heated to commonly form an elliptical galaxy.
The gas in the two components is heated by shocks
to the virial temperature of the resulting halo, which
suppresses future star formation.
Minor Mergers. If the masses of the two components in
a merger are very different, the gas of the smaller component
will basically be accreted onto the more massive
halo, where it can cool again and form new stars. By
this process, a new disk population may form. In this
model, a spiral galaxy is created by forming a bulge
in a “major merger” at earlier times, with the disk of
stars and gas being formed later in minor mergers and
by the accretion of gas. Hence the bulge of a spiral
is, in this picture, nothing but a small elliptical galaxy,
which is also suggested by the very similar characteristics
of bulges and ellipticals, including the fact that both
types of object seem to follow the same relation between
black hole mass and velocity dispersion, as explained
in Sect. 3.5.3.
The merging process of the two components does
not occur instantaneously, but since the smaller galaxy
will have, in general, a finite orbital angular momentum,
it will first enter into an orbit around the more massive
component. One example of this is the Sagittarius dwarf
galaxy, but also the Magellanic Clouds will, in a distant
future, merge with the Milky Way in this way. By dynamical
friction, the satellite galaxy then loses its orbital
397
Fig. 9.38. On the left, the distribution of dark matter resulting
from an N-body simulation is shown. The dark matter halos
identified in this mass distribution were then modeled as the
location of galaxy formation – the formation of halos and their
merger history can be followed explicitly in the simulations.
Semi-analytic models describe the processes which are most
important for the gas and the formation of stars in halos, from
which a model for the distribution of galaxies results. In the
panel on the right, the resulting distribution of model galaxies
is represented by colored dots, where the color indicates
the spectral energy distribution of the respective galaxy: blue
indicates galaxies with active star formation, red are galaxies
which are presently not forming any new stars. The latter are
particularly abundant in clusters of galaxies – in agreement
with observations