and Cosmology

9. The Universe at High Redshift
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AGN activity in their central galaxy puts out enough
energy to heat the gas and prevent it from cooling
to low temperatures. Direct and indirect evidence for
the validity of this hypothesis exists, as explained in
Sect. 6.3.3.
A similar mechanism may occur in galaxies as well.
We have learned that galaxies host a supermassive black
hole whose mass scales with the velocity dispersion of
the spheroidal stellar component and thus, in elliptical
galaxies, with the mass of the dark matter halo. If gas
accretes onto these halos, it may cool and, on the one
hand, form stars; on the other hand, this process will lead
to accretion of gas onto the central black hole and make
it active again. This activity can then heat the gas and
thus prevent further star formation. If the time needed
to cool the gas and form stars is shorter than the free-fall
time to the center of the galaxy, stars can form before
the AGN activity is switched on. In the opposite case,
star formation is prevented. A quantitative analysis of
these two time-scales shows that they are about equal
forahaloofmass∼ 2 × 10 11 h −1 M ⊙ , about the right
mass-scale for explaining the cut-off luminosity L ∗ in
the Schechter function.
Accounting for the AGN feedback explicitly in semianalytic
models of galaxy evolution provides a good
match to the observed galaxy luminosity function in
the current Universe. Furthermore, such models closely
reproduce the observed evolution of the stellar mass
function, which provides a framework for understanding
the “downsizing” phenomenon. In fact, since the
mass of the central black hole was accumulated by accretion,
and since the total energy output that can be
generated in the course of growing a black hole to a mass
of ∼ 10 8 M ⊙ is very large, it should not be too surprising
that this nuclear activity has a profound impact on
the galaxy hosting the SMBH. The fact that the hosts of
luminous QSOs show no signs of strong star formation
may be another indication that the AGN luminosity prevents
efficient star formation in its local environment.
9.7 Gamma-Ray Bursts
Discovery and Phenomenology. In 1968, surveillance
satellites for the monitoring of nuclear test ban treaties
discovered γ -flashes similar to those that are observed
in nuclear explosions. However, these satellites found
that the flashes were not directed from Earth but from
the opposite direction – hence, these γ -flashes must
be a phenomenon of cosmic origin. Since the satellite
missions were classified, the results were not published
until 1973. The sources were named gamma-ray bursts
(GRB).
The flashes are of very different duration, from a few
milliseconds up to ∼ 100 s, and they differ strongly in
their respective light curves (see Fig. 9.41). They are observed
in an energy range from ∼ 100 keV up to several
MeV, sometimes to even higher energies.
The nature of GRBs had been completely unclear
initially, because the positional accuracy of the bursts
as determined by the satellites was far too large to allow
an identification of any corresponding optical source.
The angular resolution of these γ -detectors was many
degrees (for some, a 2π solid angle). A more precise
position was determined from the time of arrival of the
bursts at the location of several satellites, but the error
box was still too large to search for counterparts of the
source in other spectral ranges.
Early Models. The model favored for a long time included
accretion phenomena on neutron stars in our
Galaxy. If their distance was D ∼ 100 pc, the corresponding
luminosity would be about L ∼ 10 38 erg/s,
thus about the Eddington luminosity of a neutron star.
Furthermore, indications of absorption lines in GRBs
at about 40 keV and 80 keV were found, which were
interpreted as cyclotron absorption corresponding to
a magnetic field of ∼ 10 12 Gauss – again, a characteristic
value for the magnetic field of neutron stars. Hence,
most researchers before the early 1990s thought that
GRBs occur in our immediate Galactic neighborhood.
The Extragalactic Origin of GRBs. A fundamental
breakthrough was then achieved with the BATSE experiment
on-board the Compton Gamma Ray Observatory,
which detected GRBs at a rate of about one per day
over a period of nine years. The statistics of these
GRBs shows that GRBs are isotropically distributed
on the sky (see Fig. 9.42), and that the flux distribution
N(> F) clearly deviates, at low fluxes, from the F −1.5 -
law. These two results meant an end to those models
that had linked GRBs to neutron stars in our Milky Way,
which becomes clear from the following argument.