Max Planck Institute for Astronomy - Annual Report 2007

FeII / MgII
unknown interactive relationship. From the new data,
these mass values can be ascertained in three different
ways:
•
•
•
14
12
10
8
6
4
2
0
0
Dietrich et al. 2003
Maiolino et. al. 2003
Freudling et. al. 2003
Iwamuro et. al. 2004
Barth et. al. 2003
This work
Iwamuro et al. 2002
Thompson et al. 1999
1 2 3
Redshift z
4 5 6
Fig. II.6.4: The Fe II/Mg II line ratio as a function of redshifting.
The five values of the recent study around z 6 are represented
in yellow by a circled x.
The most direct method consists of determining
the complete bolometric luminosity of the quasars.
If one assumes that the black hole gathers material
from its surroundings at the maximum rate possible,
then radiation pressure and gravity are held in balance.
One says then that the quasar is radiating at
its Eddington Luminosity. If one sets the Eddington
Luminosity equal to the bolometric luminosity, then
one obtains the minimal mass that black holes must
have in order to reach the observed luminosity.
A different method relies on the fact that black holes
are surrounded by a region of gas in which the quasar’s
emission lines are formed. This region is called
the Broad Line Region (BLR) because the lines of
this region are strongly broadened through rapid
rotation. The central mass can be derived from the
radius of the BLR and the velocity of the gas that
is contained therein. Both quantities can be derived,
subject to certain assumptions, from the spectrum,
especially from the width of the Mg II line. The central
mass then results from the BLR’s radius and the
velocity of the gas.
A third method depends on an empirical relation be-
tween the central mass and the continuum luminosity
at a wavelength of 135 nm and the width of the C-IV
line. Here, too, BLR characteristics are used.
All three methods were applied to the spectra of five
quasars. Because each of them contains some uncertainties,
they yield – as would be expected – different mass
values (typically different by factors of two or three),
although the CIV based method provided the largest
values in all cases. Overall, the result was a range of
300 million to 5.2 billion solar masses. The 300 million
II.6. Black Hole Activity in Quasars at High Redshift 47
solar masses represented the smallest value so far among
highly redshifted quasars. Nevertheless in most cases
very high mass values resulted, which points to a surprisingly
fast growth of supermassive black holes after
the big bang. To compare: the black hole in the center
of our galaxy has a mass of 3.6 million solar masses. To
explain this phenomenon of rapid growth is one of the
most urgent tasks of cosmology.
In this context, it would be very interesting to find
out whether, in these highly redshifted quasars, the
aforementioned correlations between the masses of the
black holes and the masses and velocity dispersions
of the bulges have validity: question that is difficult to
answer because corresponding measurements relating to
these extremely distant quasars are still rather inexact.
The mass of the bulges can be estimated with the help
of observations of molecular gas in the galaxies, such
as Dominik Riechers was able to show in 2006 (Annual
Report 2006, p. 40).
The results so far point to the fact that the redshifted
galaxies deviate from the known relation between the
masses of black holes and the masses and velocity
dispersions of the bulges. Based on these results, the
implication is that the black holes develop faster than the
galaxy bulges. Yet the results are still disputed and computer
simulations cannot yet deliver clear predictions.
If these initial suspicions are confirmed, then the following
fascinating questions follow: Did the black holes
come first and then form into galaxies? Did black holes
have the possible effect of “condensation seeds” around
which the galaxies formed? MPIA research teams are
pursuing these gripping questions and initial results are
expected in the near future.
Quasar Activity Periods
Earlier research, led by American astronomers who are
also a part of the team, of 23 quasars with redshifting
around z 6 revealed, in all cases, deep Gunn-Peterson
troughs. From this one can conclude that the intergalactic
medium up to the proximity of the quasars is
overwhelmingly neutral. At the same time it has become
clear that the reionization of the medium through
energy-rich radiation must have been a complex process
that spanned a longer period.
What quasars contributed to the reionization is a much
discussed question today. After the “switching on” of the
UV and X-ray radiation, the black hole, or rather the accretion
disk surrounding it, created around itself a consistently
expanding sphere of ionized gases in the then
(z 6) still largely neutral intergalactic medium: a socalled
Strömgren sphere, the radius of which is marked
by the long wave (red) edge of the Gunn-Peterson trough
in the continuum spectrum. Yet the determination of an
absorption edge is not simple, because the absorption is
often incomplete (Fig. II.6.5).