and Cosmology

9.6 Galaxy Formation and Evolution
Fig. 9.35. The galaxy Centaurus A. The optical image is displayed
in grayscales, the contours show the radio emission,
and in red, an infrared image is presented, taken by the ISO
satellite. The ISO map indicates the distribution of dust, which
is apparently that of a barred spiral. It seems that this elliptical
galaxy features a spiral that is stabilized by the gravitational
field of the elliptical. Presumably, this galaxy was formed in
a merger process; this may also be the reason for the AGN
activity
the centers of the galaxies can be fed, initiating AGN
activity, as is presumably seen in the galaxy Centaurus
A shown in Fig. 9.35. Due to the violence of the
interaction, part of the matter is ejected from the galaxies.
These stars and the respective gas are observable
as tidal tails in optical images or by the 21-cm emission
of neutral hydrogen. From these arguments, which
are also confirmed by numerical simulations, one expects
that in a “major merger” an elliptical galaxy may
form. In the violent interaction, the gas is either ejected,
or heated so strongly that any further star formation is
suppressed.
This scenario for the formation of ellipticals is expected
from models of structure formation. Thus far
it has been quite successful. For instance, it provides
a straightforward explanation for the Butcher–Oemler
effect (see Sect. 6.6), which states that clusters of galaxies
at higher redshift contain a larger fraction of blue
galaxies. Because of the particularly frequent mergers
in clusters, due to the high galaxy density, such blue
galaxies are transformed more and more into earlytype
galaxies. However, galaxies in clusters may also
lose their gas in their motion through the hot intergalactic
medium, by which the gas is ripped out due
to the so-called ram pressure. The fact that the fraction
of ellipticals in a cluster remains rather constant
as a function of redshift, whereas the abundance of
S0 galaxies increases with decreasing z, indicates the
importance of the latter process as an explanation of
the Butcher–Oemler effect. In this case, the gas of the
disk is stripped, no further star formation takes place,
and the spiral galaxy is changed into a disk galaxy
without any current star formation – hence, into a galaxy
that features the basic properties of S0 galaxies
(see Fig. 9.36). On the other hand, we have seen in
Sect. 3.2.5 that many ellipticals show signs of complex
evolution which can be interpreted as the consequence
of such mergers. Therefore, it is quite possible that the
formation of ellipticals in galaxy groups happens by violent
merger processes, and that these then contribute
to the cluster populations by the merging of groups into
clusters.
This model also has its problems, though. One of
these is that merger processes of galaxies are also observed
to occur at lower redshifts. Ellipticals formed
in these mergers would be relatively young, which is
hardly compatible with the above finding of a consistently
old age of ellipticals. However, ellipticals are
predominantly located in galaxy clusters whose members
are already galaxies with a low gas content. In the
merging process of such galaxies, the outcome will be
an elliptical, but no starburst will be induced by merging
because of the lack of gas – such mergers are sometimes
called “dry mergers”. In this context, we need to mention
that the phrase “age of ellipticals” refers to the age
of their stellar populations – the stars in the ellipticals
are old, but not necessarily the galaxies themselves.
The importance of dry mergers was recognized more
recently for a number of reasons. First, wide-field imaging
with HST, using mosaics of single fields, have shown
a large number of pairs of spheroidal galaxies at z 0.7
which show signs of interactions. A dramatic example
of this is also seen in Fig. 6.45, where several gravitationally
bound pairs of early-type galaxies are seen
in the outskirts of a cluster at z = 0.83. These pairs
will merge on a time-scale of 1 Gyr. Second, numer-
393