and Cosmology

9. The Universe at High Redshift
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displayed, obtained from semi-analytic models. Massive
halos which have formed early in cosmic history
are currently found predominantly in the centers of very
massive galaxy clusters. Assuming that the luminous
QSOs at z ∼ 6 are harbored in the most massive halos
of this epoch, we can deduce that these may today
be identified as the central galaxies in clusters. This
may provide an explanation as to why so many central,
dominating cluster galaxies show AGN activity,
though with a smaller luminosity due to small accretion
rates.
Abundance and Evolution of Supermassive Black
Holes. Within the framework of these models, predictions
are also made about the statistical evolution of
SMBHs in the cores of galaxies. When two galaxies
merge, their SMBHs will also coalesce after some time,
where an accurate estimate for this time-scale is difficult
to obtain. Owing to the high initial orbital angular
momentum, the two SMBHs are, at the beginning of
a merger, on an orbit with rather large mutual separation.
By dynamical friction (see Sect. 6.2.6), caused by
the matter distribution in the newly formed galaxy, the
pair of SMBHs will lose orbital energy after the merger
of the galaxies, and the two black holes will approach
each other. Since this process takes a relatively long
time, and since a massive galaxy will, besides a few
major mergers, undergo numerous minor mergers, it is
conceivable that many of the black holes that were originally
the nuclei of low-mass satellite galaxies are today
still on orbits at relatively large distances from the center
of galaxies.
In this model, phases in the evolution of galaxies exist
in which two SMBHs are located close to the center.
Indeed, there are a number of indications that such galaxies
are actually observed. For instance, galaxies with
two active nuclei have been found. Also a class of radio
sources exists with an X-shaped morphology (instead
of the usual bipolar radio structure), which can be interpreted
as pairs of active SMBHs. Another signature
of a binary system of SMBHs would be a periodicity
in the emission, reflecting the orbital period. In some
AGNs, periodic variations in the brightness have in fact
been detected, the blazar OJ 287 being the best known
example, with a period of 11.86 years.
When two SMBHs merge, the initially wide orbit
shrinks, in the final stages due to the emission of gravitational
waves. This will cause the orbits to become
more circular, as well as a decrease of the separation of
the two black holes. According to the theory of black
holes, there is a closest separation at which an orbit still
is stable. Once the separation has shrunk to that size, the
merging occurs, accompanied by a burst of gravitational
wave emission. If the two SMBHs have the same mass,
each of them will emit the same amount of gravitational
wave energy, but in opposite directions, so that the net
amount of momentum carried away by the gravitational
waves is zero. However, if the masses are not equal, this
cancellation no longer occurs, and the waves carry away
a net linear momentum. According to momentum conservation,
this will yield a recoil to the merged SMBH,
and it will therefore move out of the galactic nucleus.
Depending on the recoil velocity, it may return to the
center in a few dynamical time-scales. However, if the
recoil velocity is larger than the escape velocity from
the galaxy, it may actually escape from the gravitational
potential and become an intergalactic black hole. The
importance of this effect is not quantitatively known,
since the amplitude of the recoil velocity as determined
from theoretical models is uncertain. It is zero for equal
masses, and very small if one of the black hole masses
is much smaller than the other. The recoil velocity attains
a maximum value when the mass ratio of the two
SMBHs is about 1/3.
The mass increase of SMBHs in the course of cosmic
history then has two different origins, first the merging
with other low-mass SMBHs as a consequence of
merger events, and second the accretion of gas that
leads to the activity of SMBHs. Hierarchical models
of galaxy evolution with central SMBHs are able to
both reproduce, under certain assumptions, the correlation
(see Sect. 3.5.3) between the SMBH mass and
the properties of the spheroidal stellar component, and
to successfully model the integrated AGN luminosity
and the redshift-dependent luminosity function of
AGNs.
In the course of the merger of two SMBHs, an intense
emission of gravitational waves will occur in the final
phase. The space project LISA, which is planned to be
launched sometime after 2013, is capable of observing
the emission of gravitational waves from such merger
processes, essentially throughout the visible Universe.
Hence, it will become possible to directly trace the
merger history of galaxies.