Max Planck Institute for Astronomy - Annual Report 2007

limits because the observed rest wavelength interval lies
in the ultraviolet in which only regions of strong star
formation, but no old stellar populations, can be seen.
After subtracting the quasar core, the galaxy images frequently
show asymmetrical, and in rare cases strongly
disturbed, systems. An initial comparative analysis with
inactive normal galaxies shows no significant over-frequency
of disturbances in quasars with these midlevel
luminosities.
The (M BH : M Gal ) Relationship between the Masses of
the Black Hole and of the Bulge
In the meantime it has been broadly established, which
galaxies are homes to quasars: All mass-rich galaxies
with a bulge possess a central mass-rich black hole and
can appear in a quasar phase or are likely already in one.
The only requirement is the existence of accretable gas
which must reach the center. The exact physical processes
which lead to the accretion from the galaxy into
its central black hole remain unclear. Simulations of the
M BH M Gal relationship are not based on fundamental
physics but continue to contain ad hoc assumptions.
One of the best diagnostic possibilities to constrain
existing models is to examinate how the relationship
between black hole and galaxy bulge masses
changed over cosmic time. Varying physical mechanisms
are required to, for example, maintain the constant
M BH : M Gal 1 : 700 ratio observed in the local
universe or to allow for the earlier evolution of black
holes or galaxies. It is already difficult to determine
this M BH : M Gal ratio in the local Universe and at larger
redshts it turns out to be a real challenge. At larger redshifts
the only galaxies in which the mass of the black
hole can be determined are quasar host galaxies. The
quasarʼs broad emission lines permit, in combination
with its luminosity, an M BH : M Gal estimate within a factor
of 3 – 4. This presupposes, however, that the galaxies
can only be analyzed with the complications for
spectroscopy described above which equally apply to
imaging data.
In one of the first and best attempts in this direction,
the luminosity of quasars of host galaxies enhanced by
the gravitational lens effect are used to estimate their
masses subject to assumptions about their stellar populations
(Fig. III.2.6). Up to z 1.7, no strong indications
of a change in the M BH : M Gal ratio are found. The ratio
Fig. III.2.5: Quasars and their host galaxies taken with the
HST as part of the co S m o S project. This selection of 24 of
of approx. 300 quasars shows objects at redshifts z 0.35
(top) to z 2 (bottom). In each image pair, the left one is the
original image and the right one shows the host galaxy after
removal of a point source. The quasars were taken in I-Band
with the HST ACS/WFC camera. (Jahnke et co S m o S et al.
2008, in prep.)
III.2 In What Type of Galaxies Do Quasars III.X Live? Titel 75