and Cosmology

3.5 Black Holes in the Centers of Galaxies
ing is the observation of the rotation curve very close
to the center. Another impressive example is the central
region of M87, the central galaxy of the Virgo Cluster.
The increase of the rotation curve and the broadening
of the [OII]-line (a spectral line of singly-ionized oxygen)
at λ = 3727 Å towards the center are displayed in
Fig. 3.26 and argue very convincingly for a SMBH with
M • ≈ 3 × 10 9 M ⊙ .
The mapping of the Kepler rotation in the center of
the Seyfert galaxy NGC 4258 is especially spectacular.
This galaxy contains water masers – very compact
sources whose position can be observed with very high
precision using VLBI techniques (Fig. 3.27). In this
case, the deviation from a Kepler rotation in the gravitational
field of a point mass of M • ∼ 3.5 × 10 7 M ⊙ is
much less than 1%. The maser sources are embedded in
an accretion disk having a thickness of less than 0.3% of
its radius, of which also a warping is detected. Changes
in the radial velocities and the proper motions of these
maser sources have already been measured, so that the
model of a Kepler accretion disk has been confirmed in
detail.
All these observations are of course no proof of the
existence of a SMBH in these galaxies because the
sources from which we obtain the kinematic evidence
are still too far away from the Schwarzschild radius. The
conclusion of the presence of SMBHs is rather that of
a missing alternative, as was already explained for the
case of the GC (Sect. 2.6.3). We have no other plausible
model for the mass concentrations detected. As for the
case of the SMBH in the Milky Way, an ultra-compact
star cluster might be postulated, but such a cluster would
not be stable over a long period of time. Based on the
existence of a SMBH in our Galaxy and in AGNs, the
SMBH hypothesis is the only plausible explanation for
these mass concentrations.
3.5.3 Correlation Between SMBH Mass
and Galaxy Properties
Currently, strong indications of SMBHs have been
found in about 35 normal galaxies, and their masses
have been estimated. This permits us to examine
whether, and in what way, M • is related to the properties
of the host galaxy. This leads us to the discovery of a remarkable
correlation; it is found that M • is correlated
with the absolute magnitude of the bulge component
(or the spheroidal component) of the galaxy in which
111
Fig. 3.24. An HST image of the nucleus of the galaxy M84 is
shownintheleft-handpanel.M84isamemberoftheVirgo
Cluster, about 15 Mpc away from us. The small rectangle depicts
the position of the slit used by the STIS (Space Telescope
Imaging Spectrograph) instrument on-board the HST to obtain
a spectrum of the central region. This long-slit spectrum
is shown in the right-hand panel; the position along the slit
is plotted vertically, the wavelength of the light horizontally,
also illustrated by colors. Near the center of the galaxy the
wavelength suddenly changes because the rotational velocity
steeply increases inwards and then changes sign on the other
side of the center. This shows the Kepler rotation in the central
gravitational field of a SMBH, whose mass can be estimated
as M • ∼ 3 × 10 8 M ⊙