Black Holes: from Stars to Galaxies – across the Range of Masses
Proceedings IAU Symposium No. 238, 2006
V. Karas & G. Matt, eds.
c© 2006 International Astronomical Union
DOI: 10.1017/S1743921307004851
The Galactic Center
Reinhard Genzel1,2 and Vladimı́r Karas3
1 Max-Planck Institut für Extraterrestrische Physik, Garching, Germany
2 Department of Physics, University of California, Berkeley, USA
3 Astronomical Institute, Academy of Sciences, Prague, Czech Republic
Abstract. In the past decade high resolution measurements in the infrared employing adaptive
optics imaging on 10m telescopes have allowed determining the three dimensional orbits stars
within ten light hours of the compact radio source at the center of the Milky Way. These
observations show the presence of a three million solar mass black hole in Sagittarius A* beyond
any reasonable doubt. The Galactic Center thus constitutes the best astrophysical evidence
for the existence of black holes which have long been postulated, and is also an ideal ‘lab’ for
studying the physics in the vicinity of such an object. Remarkably, young massive stars are
present there and probably have formed in the innermost stellar cusp. Variable infrared and
X-ray emission from Sagittarius A* are a new probe of the physical processes and space-time
curvature just outside the event horizon.
Keywords. Galaxy: center – black hole physics
1. Introduction – Sagittarius A*
The central light years of our Galaxy contain a dense and luminous star cluster, as
well as several components of neutral, ionized and extremely hot gas (Genzel, Hollenbach
& Townes 1994). The Galactic Center also contains a very compact radio source, Sagit-
tarius A* (Sgr A*; Balick & Brown 1974) which is located at the center of the nuclear
star cluster and ionized gas environment. Short-wavelength centimeter and millimeter
VLBI observations have established that its intrinsic radio size is a mere 10 light min-
utes (Bower et al 2004; Shen et al 2005). Sgr A* is also an X-ray emission source, albeit
of only modest luminosity (Baganoff et al 2001). Most recently, Aharonian et al (2004)
have discovered a source of TeV γ-ray emission within 10 arcsec of Sgr A*. It is not yet
clear whether these most energetic γ-rays come from Sgr A* itself or whether they are
associated with the nearby supernova remnant, Sgr A East.
Sgr A* thus may be a supermassive black hole analogous to QSOs, albeit of much
lower mass and luminosity. Because of its proximity – the distance to the Galactic Center
is about 105 times closer than the nearest quasars – high resolution observations of the
Milky Way nucleus offer the unique opportunity of stringently testing the black hole
paradigm and of studying stars and gas in the immediate vicinity of a black hole, at a
level of detail that will not be accessible in any other galactic nucleus in the foreseeable
future.
Since the center of the Milky Way is highly obscured by interstellar dust particles in
the plane of the Galactic disk, observations in the visible light are not possible. Investi-
gations require measurements at longer wavelengths – the infrared and microwave bands,
or at shorter wavelengths – hard X-rays and γ-rays, where the veil of dust is transparent.
The dramatic progress in our knowledge of the Galactic Center over the past two decades
is a direct consequence of the development of novel facilities, instruments and techniques
across the whole range of the electromagnetic spectrum.
http://arxiv.org/abs/0704.1281v1
174 R. Genzel & V. Karas
Figure 1. Left: VLA radio continuum map of the central parsec (Roberts & Goss 1993). The
radio emission delineates ionized gaseous streams orbiting the compact radio source Sgr A*.
Spectroscopic measurements in the radio band (Wollman et al 1977) provided the first dynamical
evidence from large gas velocities that there might be a hidden mass of 3–4 million solar masses
located near Sgr A*. Right: A diffraction limited image of Sgr A* (∼ 0.05 arcsec resolution)
from the 8m ESO VLT, taken with the NACO AO-camera and an infrared wavefront sensor
at 1.6/2.2/3.7 µm (Genzel et al 2003b). The central black hole is located in the centre of the
box. NACO is a collaboration between ONERA (Paris), Observatoire de Paris, Observatoire
Grenoble, MPE (Garching), and MPIA (Heidelberg) (Lenzen et al 1998; Rousset et al 1998).
2. High angular resolution astronomy
The key to the nature of Sgr A* obviously lies in very high angular resolution
measurements. The Schwarzschild radius of a 3.6 million solar mass black hole at the
Galactic Center subtends a mere 10−5 arcsec. For the high-resolution imaging from the
ground it is necessary to correct for the distortions of an incoming electromagnetic wave
by the refractive and dynamic Earth atmosphere. VLBI overcomes this hurdle by phase-
referencing to nearby QSOs; sub-milliarcsecond resolution can now be routinely achieved.
In the optical/near-infrared wavebands the atmosphere smears out long-exposure
images to a diameter at least ten times greater than the diffraction limited resolution
of large ground-based telescopes (Fig. 1). From the early 1990s onward initially speckle
imaging (recording short exposure images, which are subsequently processed and co-
added to retrieve the diffraction limited resolution) and then later adaptive optics (AO,
correcting the wave distortions on-line) became available. With these techniques it is
possible to achieve diffraction limited resolution on large ground-based telescopes. The
diffraction limited images are much sharper and also much deeper than the seeing limited
images. In the case of AO (Beckers 1993) the incoming wavefront of a bright star near
the source of interest is analyzed, the necessary corrections for undoing the aberrations
of the atmosphere are computed (on time scales shorter than the atmospheric coherence
time of a few milli-seconds) and these corrections are then applied to a deformable optical
element (e.g. a mirror) in the light path.
The requirements on the brightness of the AO star and on the maximum allowable
separation between star and source are quite stringent, resulting in a very small sky
coverage of natural star AO. Fortunately, in the Galactic Center there is a bright infrared
star only 6 arcsec away from Sgr A*, such that good AO correction can be achieved
The Galactic Center 175
with an infrared wavefront sensor system. Artificial laser beacons can overcome the sky
coverage problem to a considerable extent. For this purpose, a laser beam is projected
from the telescope into the upper atmosphere and the backscattered laser light can then
be used for AO correction. The Keck telescope team has already begun successfully
exploiting the new laser guide star technique for Galactic Center research (Ghez et al
2005a). After AO correction, the images are an order of magnitude sharper and also
much deeper than in conventional seeing limited measurements. The combination of
AO techniques with advanced imaging and spectroscopic instruments (e.g. integral field
imaging spectroscopy) have resulted in a major breakthrough in high resolution studies
of the Galactic Center.
3. Nuclear star cluster and the paradox of youth
One of the big surprises is a fairly large number of bright stars in Sgr A*, a number of
which were already apparent on the discovery infrared images of Becklin & Neugebauer
(1975, 1978). High-resolution infrared spectroscopy reveals that many of these bright
stars are actually somewhat older, late-type supergiants and AGB stars. Starting with
the discovery of the AF-star (Allen et al 1990; Forrest et al 1987), however, an ever
increasing number of the bright stars have been identified as being young, massive and
early type. The most recent counts from the deep SINFONI integral-field spectroscopy
yields about one hundred OB stars, including various luminous blue supergiants and
Wolf-Rayet stars, but also normal main-sequence OB stars (Paumard et al 2006a). The
nuclear star cluster is one of the richest concentrations of young massive stars in the
Milky Way.
The deep adaptive optics images also trace the surface density distribution of the
fainter stars, to about K 17–18 mag, corresponding to late B or early A stars (masses of
3–6 solar masses), which are a better probe of the density distribution of the overall mass
density of the star cluster. While the surface brightness distribution of the star cluster
is not centered on Sgr A*, the surface density distribution is. There is clearly a cusp
of stars centered on the compact radio source (Genzel et al 2003b; Schödel et al 2006).
The inferred volume density of the cusp is a power-law ∝R−1.4±0.1, consistent with the
expectation for a stellar cusp around a massive black hole (Alexander 2005).
If there is indeed a central black hole associated with Sgr A* the presence of so many
young stars in its immediate vicinity constitutes a significant puzzle (Allen & Sanders
1986; Morris 1993; Alexander 2005). For gravitational collapse to occur in the presence of
the tidal shear from the central mass, gas clouds have to be denser than∼ 109(R/(10′′))−3
hydrogen atoms per cm−3. This ‘Roche’ limit exceeds the density of any gas currently
observed in the central region. Recent near-diffraction limited AO spectroscopy with both
the Keck and VLT shows that almost all of the cusp stars brighter than K ∼ 16 mag
appear to be normal, main sequence B stars (Ghez et al 2003; Eisenhauer et al 2005a). If
these stars formed in situ, the required cloud densities approach the conditions in outer
stellar atmospheres.
Several scenarios have been proposed to account for this paradox of youth. In spite
of this effort the origin of central stars (S-stars) is not well understood: models have
difficulties in reconciling different aspects of the Galaxy Centre – on one side it is a low
level of present activity, indicating a very small accretion rate, and on the other side it is
the spectral classification that suggests these stars have been formed relatively recently;
see Alexander (2005) for a detailed discussion and references. The most prominent ideas
to resolve the apparent problem are in situ formation in a dense gas accretion disk that
can overcome the tidal limits, re-juvenation of older stars by collisions or stripping, and
176 R. Genzel & V. Karas
Figure 2. Positions on the sky as a function of time for the central stars orbiting the compact
radio source Sgr A*. Left: the data from the UCLA group working with the Keck telescope
(Ghez et al 2005b). Right: the data from the MPE–Cologne group at the ESO-VLT (Schödel
et al 2003; Eisenhauer et al 2005a; Gillessen et al, in preparation).
rapid in-spiral of a compact, massive star cluster that formed outside the central region
and various scattering a three body interaction mechanisms, including resonant relaxation
(Alexander 2005). Several other mechanisms have been proposed that could set stars on
highly eccentric orbits and bring them to the neighbourhood of the central black hole
(e.g., Hansen & Milosavljević 2003; McMillan & Portegies Zwart 2003; Alexander & Livio
2004; Šubr & Karas 2005), but the problem of the S-stars remains open.
4. Compelling evidence for a central massive black hole
With diffraction limited imagery starting in 1991 on the 3.5m ESO New Technology
Telescope and continuing since 2002 on the VLT, a group at MPE was able to determine
proper motions of stars as close as∼ 0.1 arcsec from Sgr A* (Eckart & Genzel 1996, 1997).
In 1995 a group at the University of California, Los Angeles started a similar program
with the 10m diameter Keck telescope (Ghez et al 1998). Both groups independently
found that the stellar velocities follow Kepler laws and exceed 103 km/s within the
central light month.
Only a few years later both groups achieved the next and crucial step: they were
able to determine individual stellar orbits for several stars very close to the compact
radio source (Fig. 2; Schödel et al. 2002, 2003; Ghez et al 2003, 2005b; Eisenhauer et al
2005a). In addition to the astrometric imaging they obtained near-diffraction limited
Doppler spectroscopy of the same stars (Ghez et al 2003; Eisenhauer et al 2003a,b),
yielding precision measurements of the three dimensional structure of several orbits, as
well as the distance to the Galactic Center. At the time of writing, the orbits have been
determined for about a dozen stars in the central light month. The central mass and
stellar orbital parameters derived by the two teams agree mostly very well. The orbits
show that the gravitational potential indeed is that of a point mass centered on Sgr A*
within the relative astrometric uncertainties of ∼ 10 milliarcsec. Most of the mass must
be concentrated well within the peri-approaches of the innermost stars, ∼ 10–20 light
The Galactic Center 177
hours, or 70 times the Earth orbit radius and about 1000 times the event horizon of a
3.6 million solar mass black hole. There is presently no indication for an extended mass
greater than about 5% of the point mass.
Simulations indicate that current measurement accuracies are sufficient to reveal
the first and second order effects of Special and General Relativity in a few years time
(Zucker et al 2006). Observations with future 30m+ diameter telescopes will be able
to measure the mass and distance to the Galactic Center to ∼ 0.1% precision. They
should detect radial precession of stellar orbits due to General Relativity and constrain
the extended mass to < 10−3 of the massive black hole (Weinberg, Milosavljevic &
Ghez 2005). At that level a positive detection of a halo of stellar remnants (stellar black
holes and neutron stars) and perhaps dark matter would appear to be likely. Future
interferometric techniques will push capabilities yet further.
Long-term VLBA observations have set 2σ upper limits of about 20 km/s and 2 km/s
(or 50 micro-arcsec per year) to the motion of Sgr A* itself, along and perpendicular
to the plane of the Milky Way, respectively (Reid & Brunthaler 2004; see also Backer
& Sramek 1999). This precision measurement demonstrates very clearly that the radio
source itself must indeed be massive, with simulations indicating a lower limit to the
mass of Sgr A* of ∼ 105 solar masses. The intrinsic size of the radio source at millimeter
wavelengths is less than 5 to 20 times the event horizon diameter (Bower et al 2004;
Shen et al 2005). Combining the radio size and proper motion limit of Sgr A* with the
dynamical measurements of the nearby orbiting stars leads to the conclusion that Sgr A*
can only be a massive black hole, beyond any reasonable doubt. An astrophysical dark
cluster fulfilling the observational constraints would have a life-time less than a few 104
years and thus can be safely rejected, as can be a possible fermion ball of hypothetical
heavy neutrinos. In fact all non-black hole configurations can be excluded by the available
measurements (Schödel et al 2003; Ghez et al 2005b) – except for a hypothetical boson
star and the gravastar hypothesis, but it appears that the two mentioned alternatives
have difficulties of their own, and they are less likely and certainly much less understood
than black holes (e.g. Maoz 1998; Miller et al 1998). We thus conclude that, under the
assumption of the validity of General Relativity, the Galactic Center provides the best
quantitative evidence for the actual existence of (massive) black holes that contemporary
astrophysics can offer.
5. Zooming in on the accretion zone and event horizon
Recent millimeter, infrared and X-ray observations have detected irregular, and
sometimes intense outbursts of emission from Sgr A* lasting anywhere between 30 min-
utes and a number of hours and occurring at least once per day (Baganoff et al 2001;
Genzel et al 2003a; Marrone et al 2006). These flares originate from within a few milli-
arcseconds of the radio position of Sgr A*. They probably occur when relativistic elec-
trons in the innermost accretion zone of the black hole are significantly accelerated, so
that they are able to produce infrared synchrotron emission and X-ray synchrotron or
inverse Compton radiation (Markoff et al 2001; Yuan et al 2003; Liu et al 2005). This
interpretation is also supported by the detection of significant polarization of the infrared
flares (Eckart et al 2006b), by the simultaneous occurrence of X- and IR-flaring activity
(Eckart et al 2006a; Yusef-Zadeh et al. 2006) and by variability in the infrared spectral
properties (Ghez et al 2005b; Gillessen et al 2006a; Krabbe et al. 2006). There are in-
dications for quasi-periodicities in the light curves of some of these flares, perhaps due
to orbital motion of hot gas spots near the last circular orbit around the event horizon
(Genzel et al 2003a; Aschenbach et al 2004; Bélanger et al 2006).
178 R. Genzel & V. Karas
Figure 3. Photo-center wobbling (left) and light curve (right) of a hot spot on the innermost
stable orbit around Schwarzschild black hole (inclination of 80 deg), as derived from ray-tracing
computations. Dotted curve: ‘true’ path of the hot spot; dashed curves: apparent path and a pre-
dicted light curve of the primary image; dash-dotted curves: the same for secondary image; solid
curves: path of centroid and integrated light curve. Axes on the left panel are in Schwarzschild
radii of a 3 million solar-mass black hole, roughly equal to the astrometric accuracy of 10 arcsec;
the abscissa axis of the right panel is in cycles. The loop in the centroids track is due to the
secondary image, which is strongly sensitive to the space-time curvature. The overall motion
can be detected at good significance at the anticipated accuracy of GRAVITY. Details can be
obtained by analyzing several flares simultaneously (Gillessen et al 2006b; Paumard et al 2005).
The infrared flares as well as the steady microwave emission from Sgr A* may be
important probes of the gas dynamics and space-time metric around the black hole
(Broderick & Loeb 2006; Meyer et al 2006a,b; Paumard et al 2006b). Future long-baseline
interferometry at short millimeter or sub-millimeter wavelengths may be able to map out
the strong light-bending effects around the photon orbit of the black hole. It is interesting
to realize that the angular size of the “shadow” of black hole (Bardeen 1973) is not very
far from the anticipated resolution of interferometric techniques and it may thus be
accessible to observations in near future (Falcke, Melia & Agol 2000).
Polarization measurements will help us to set further constraints on the emission
processes responsible for the flares. Especially the time-resolved lightcurves of the polar-
ized signal carry specific information about the interplay between the gravitational and
magnetic fields near Sgr A* horizon, because the propagation of the polarization vec-
tor is sensitive to the presence and properties of these fields along the light trajectories
(Bromley, Melia & Liu 2001; Horák & Karas 2006; Paumard et al 2006b). Polarization
is also very sensitive also to intrinsic properties of the source – its geometry and details
of radiation mechanisms responsible for the emission.
Synthesis of different techniques will be a promising way for the future: the as-
trometry of central stars gives very robust results because the stellar motion is almost
unaffected by poorly known processes of non-gravitational origin, while the flaring gas
occurs much closer to the black hole horizon and hence it directly probes the innermost
regions of Sgr A*. Eventually the two components – gas and stars of the Galaxy Center –
are interconnected and form the unique environment in which the flaring gas is influenced
by intense stellar winds whereas the long-term motion and the ‘non-standard’ evolution
of the central stars bears imprints of the gaseous medium though which the stars pass.
Eisenhauer et al (2005b) are developing GRAVITY (an instrument for ‘General
Relativity Analysis via VLT Interferometry’), which will provide dual-beam, 10 micro-
arcsecond precision infrared astrometric imaging of faint sources. GRAVITY may be able
to map out the motion on the sky of hot spots during flares with a high enough resolu-
The Galactic Center 179
tion and precision to determine the size of the emission region and possibly detect the
imprint of multiple gravitational images (see Fig. 3). In addition to studies of the flares,
it will also be able to image the orbits of stars very close to the black hole, which should
then exhibit the orbital radial oscillations and Lense-Thirring precession due to General
Relativity. Both the microwave shadows as well as the infrared hot spots are sensitive to
the space-time metric in the strong gravity regime. As such, these ambitious future ex-
periments can potentially test the validity of the black hole model near the event horizon
and perhaps even the validity of General Relativity in the strong field limit.
Acknowledgements
An extended version of this lecture was presented by RG as Invited Discourse during
the 26th General Assembly of the International Astronomical Union in Prague, 22nd
August 2006 (Highlights of Astronomy, Volume 14, 2007). VK thanks the Czech Science
Foundation for continued support (ref. 205/07/0052).
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	Introduction – Sagittarius A*
	High angular resolution astronomy
	Nuclear star cluster and the paradox of youth
	Compelling evidence for a central massive black hole
	Zooming in on the accretion zone and event horizon

  In the past decade high resolution measurements in the infrared employing
adaptive optics imaging on 10m telescopes have allowed determining the three
dimensional orbits stars within ten light hours of the compact radio source at
the center of the Milky Way. These observations show the presence of a three
million solar mass black hole in Sagittarius A* beyond any reasonable doubt.
The Galactic Center thus constitutes the best astrophysical evidence for the
existence of black holes which have long been postulated, and is also an ideal
`lab' for studying the physics in the vicinity of such an object. Remarkably,
young massive stars are present there and probably have formed in the innermost
stellar cusp. Variable infrared and X-ray emission from Sagittarius A* are a
new probe of the physical processes and space-time curvature just outside the
event horizon.

Introduction – Sagittarius A*
The central light years of our Galaxy contain a dense and luminous star cluster, as
well as several components of neutral, ionized and extremely hot gas (Genzel, Hollenbach
& Townes 1994). The Galactic Center also contains a very compact radio source, Sagit-
tarius A* (Sgr A*; Balick & Brown 1974) which is located at the center of the nuclear
star cluster and ionized gas environment. Short-wavelength centimeter and millimeter
VLBI observations have established that its intrinsic radio size is a mere 10 light min-
utes (Bower et al 2004; Shen et al 2005). Sgr A* is also an X-ray emission source, albeit
of only modest luminosity (Baganoff et al 2001). Most recently, Aharonian et al (2004)
have discovered a source of TeV γ-ray emission within 10 arcsec of Sgr A*. It is not yet
clear whether these most energetic γ-rays come from Sgr A* itself or whether they are
associated with the nearby supernova remnant, Sgr A East.
Sgr A* thus may be a supermassive black hole analogous to QSOs, albeit of much
lower mass and luminosity. Because of its proximity – the distance to the Galactic Center
is about 105 times closer than the nearest quasars – high resolution observations of the
Milky Way nucleus offer the unique opportunity of stringently testing the black hole
paradigm and of studying stars and gas in the immediate vicinity of a black hole, at a
level of detail that will not be accessible in any other galactic nucleus in the foreseeable
future.
Since the center of the Milky Way is highly obscured by interstellar dust particles in
the plane of the Galactic disk, observations in the visible light are not possible. Investi-
gations require measurements at longer wavelengths – the infrared and microwave bands,
or at shorter wavelengths – hard X-rays and γ-rays, where the veil of dust is transparent.
The dramatic progress in our knowledge of the Galactic Center over the past two decades
is a direct consequence of the development of novel facilities, instruments and techniques
across the whole range of the electromagnetic spectrum.
http://arxiv.org/abs/0704.1281v1
174 R. Genzel & V. Karas
Figure 1. Left: VLA radio continuum map of the central parsec (Roberts & Goss 1993). The
radio emission delineates ionized gaseous streams orbiting the compact radio source Sgr A*.
Spectroscopic measurements in the radio band (Wollman et al 1977) provided the first dynamical
evidence from large gas velocities that there might be a hidden mass of 3–4 million solar masses
located near Sgr A*. Right: A diffraction limited image of Sgr A* (∼ 0.05 arcsec resolution)
from the 8m ESO VLT, taken with the NACO AO-camera and an infrared wavefront sensor
at 1.6/2.2/3.7 µm (Genzel et al 2003b). The central black hole is located in the centre of the
box. NACO is a collaboration between ONERA (Paris), Observatoire de Paris, Observatoire
Grenoble, MPE (Garching), and MPIA (Heidelberg) (Lenzen et al 1998; Rousset et al 1998).
2. High angular resolution astronomy
The key to the nature of Sgr A* obviously lies in very high angular resolution
measurements. The Schwarzschild radius of a 3.6 million solar mass black hole at the
Galactic Center subtends a mere 10−5 arcsec. For the high-resolution imaging from the
ground it is necessary to correct for the distortions of an incoming electromagnetic wave
by the refractive and dynamic Earth atmosphere. VLBI overcomes this hurdle by phase-
referencing to nearby QSOs; sub-milliarcsecond resolution can now be routinely achieved.
In the optical/near-infrared wavebands the atmosphere smears out long-exposure
images to a diameter at least ten times greater than the diffraction limited resolution
of large ground-based telescopes (Fig. 1). From the early 1990s onward initially speckle
imaging (recording short exposure images, which are subsequently processed and co-
added to retrieve the diffraction limited resolution) and then later adaptive optics (AO,
correcting the wave distortions on-line) became available. With these techniques it is
possible to achieve diffraction limited resolution on large ground-based telescopes. The
diffraction limited images are much sharper and also much deeper than the seeing limited
images. In the case of AO (Beckers 1993) the incoming wavefront of a bright star near
the source of interest is analyzed, the necessary corrections for undoing the aberrations
of the atmosphere are computed (on time scales shorter than the atmospheric coherence
time of a few milli-seconds) and these corrections are then applied to a deformable optical
element (e.g. a mirror) in the light path.
The requirements on the brightness of the AO star and on the maximum allowable
separation between star and source are quite stringent, resulting in a very small sky
coverage of natural star AO. Fortunately, in the Galactic Center there is a bright infrared
star only 6 arcsec away from Sgr A*, such that good AO correction can be achieved
The Galactic Center 175
with an infrared wavefront sensor system. Artificial laser beacons can overcome the sky
coverage problem to a considerable extent. For this purpose, a laser beam is projected
from the telescope into the upper atmosphere and the backscattered laser light can then
be used for AO correction. The Keck telescope team has already begun successfully
exploiting the new laser guide star technique for Galactic Center research (Ghez et al
2005a). After AO correction, the images are an order of magnitude sharper and also
much deeper than in conventional seeing limited measurements. The combination of
AO techniques with advanced imaging and spectroscopic instruments (e.g. integral field
imaging spectroscopy) have resulted in a major breakthrough in high resolution studies
of the Galactic Center.
3. Nuclear star cluster and the paradox of youth
One of the big surprises is a fairly large number of bright stars in Sgr A*, a number of
which were already apparent on the discovery infrared images of Becklin & Neugebauer
(1975, 1978). High-resolution infrared spectroscopy reveals that many of these bright
stars are actually somewhat older, late-type supergiants and AGB stars. Starting with
the discovery of the AF-star (Allen et al 1990; Forrest et al 1987), however, an ever
increasing number of the bright stars have been identified as being young, massive and
early type. The most recent counts from the deep SINFONI integral-field spectroscopy
yields about one hundred OB stars, including various luminous blue supergiants and
Wolf-Rayet stars, but also normal main-sequence OB stars (Paumard et al 2006a). The
nuclear star cluster is one of the richest concentrations of young massive stars in the
Milky Way.
The deep adaptive optics images also trace the surface density distribution of the
fainter stars, to about K 17–18 mag, corresponding to late B or early A stars (masses of
3–6 solar masses), which are a better probe of the density distribution of the overall mass
density of the star cluster. While the surface brightness distribution of the star cluster
is not centered on Sgr A*, the surface density distribution is. There is clearly a cusp
of stars centered on the compact radio source (Genzel et al 2003b; Schödel et al 2006).
The inferred volume density of the cusp is a power-law ∝R−1.4±0.1, consistent with the
expectation for a stellar cusp around a massive black hole (Alexander 2005).
If there is indeed a central black hole associated with Sgr A* the presence of so many
young stars in its immediate vicinity constitutes a significant puzzle (Allen & Sanders
1986; Morris 1993; Alexander 2005). For gravitational collapse to occur in the presence of
the tidal shear from the central mass, gas clouds have to be denser than∼ 109(R/(10′′))−3
hydrogen atoms per cm−3. This ‘Roche’ limit exceeds the density of any gas currently
observed in the central region. Recent near-diffraction limited AO spectroscopy with both
the Keck and VLT shows that almost all of the cusp stars brighter than K ∼ 16 mag
appear to be normal, main sequence B stars (Ghez et al 2003; Eisenhauer et al 2005a). If
these stars formed in situ, the required cloud densities approach the conditions in outer
stellar atmospheres.
Several scenarios have been proposed to account for this paradox of youth. In spite
of this effort the origin of central stars (S-stars) is not well understood: models have
difficulties in reconciling different aspects of the Galaxy Centre – on one side it is a low
level of present activity, indicating a very small accretion rate, and on the other side it is
the spectral classification that suggests these stars have been formed relatively recently;
see Alexander (2005) for a detailed discussion and references. The most prominent ideas
to resolve the apparent problem are in situ formation in a dense gas accretion disk that
can overcome the tidal limits, re-juvenation of older stars by collisions or stripping, and
176 R. Genzel & V. Karas
Figure 2. Positions on the sky as a function of time for the central stars orbiting the compact
radio source Sgr A*. Left: the data from the UCLA group working with the Keck telescope
(Ghez et al 2005b). Right: the data from the MPE–Cologne group at the ESO-VLT (Schödel
et al 2003; Eisenhauer et al 2005a; Gillessen et al, in preparation).
rapid in-spiral of a compact, massive star cluster that formed outside the central region
and various scattering a three body interaction mechanisms, including resonant relaxation
(Alexander 2005). Several other mechanisms have been proposed that could set stars on
highly eccentric orbits and bring them to the neighbourhood of the central black hole
(e.g., Hansen & Milosavljević 2003; McMillan & Portegies Zwart 2003; Alexander & Livio
2004; Šubr & Karas 2005), but the problem of the S-stars remains open.
4. Compelling evidence for a central massive black hole
With diffraction limited imagery starting in 1991 on the 3.5m ESO New Technology
Telescope and continuing since 2002 on the VLT, a group at MPE was able to determine
proper motions of stars as close as∼ 0.1 arcsec from Sgr A* (Eckart & Genzel 1996, 1997).
In 1995 a group at the University of California, Los Angeles started a similar program
with the 10m diameter Keck telescope (Ghez et al 1998). Both groups independently
found that the stellar velocities follow Kepler laws and exceed 103 km/s within the
central light month.
Only a few years later both groups achieved the next and crucial step: they were
able to determine individual stellar orbits for several stars very close to the compact
radio source (Fig. 2; Schödel et al. 2002, 2003; Ghez et al 2003, 2005b; Eisenhauer et al
2005a). In addition to the astrometric imaging they obtained near-diffraction limited
Doppler spectroscopy of the same stars (Ghez et al 2003; Eisenhauer et al 2003a,b),
yielding precision measurements of the three dimensional structure of several orbits, as
well as the distance to the Galactic Center. At the time of writing, the orbits have been
determined for about a dozen stars in the central light month. The central mass and
stellar orbital parameters derived by the two teams agree mostly very well. The orbits
show that the gravitational potential indeed is that of a point mass centered on Sgr A*
within the relative astrometric uncertainties of ∼ 10 milliarcsec. Most of the mass must
be concentrated well within the peri-approaches of the innermost stars, ∼ 10–20 light
The Galactic Center 177
hours, or 70 times the Earth orbit radius and about 1000 times the event horizon of a
3.6 million solar mass black hole. There is presently no indication for an extended mass
greater than about 5% of the point mass.
Simulations indicate that current measurement accuracies are sufficient to reveal
the first and second order effects of Special and General Relativity in a few years time
(Zucker et al 2006). Observations with future 30m+ diameter telescopes will be able
to measure the mass and distance to the Galactic Center to ∼ 0.1% precision. They
should detect radial precession of stellar orbits due to General Relativity and constrain
the extended mass to < 10−3 of the massive black hole (Weinberg, Milosavljevic &
Ghez 2005). At that level a positive detection of a halo of stellar remnants (stellar black
holes and neutron stars) and perhaps dark matter would appear to be likely. Future
interferometric techniques will push capabilities yet further.
Long-term VLBA observations have set 2σ upper limits of about 20 km/s and 2 km/s
(or 50 micro-arcsec per year) to the motion of Sgr A* itself, along and perpendicular
to the plane of the Milky Way, respectively (Reid & Brunthaler 2004; see also Backer
& Sramek 1999). This precision measurement demonstrates very clearly that the radio
source itself must indeed be massive, with simulations indicating a lower limit to the
mass of Sgr A* of ∼ 105 solar masses. The intrinsic size of the radio source at millimeter
wavelengths is less than 5 to 20 times the event horizon diameter (Bower et al 2004;
Shen et al 2005). Combining the radio size and proper motion limit of Sgr A* with the
dynamical measurements of the nearby orbiting stars leads to the conclusion that Sgr A*
can only be a massive black hole, beyond any reasonable doubt. An astrophysical dark
cluster fulfilling the observational constraints would have a life-time less than a few 104
years and thus can be safely rejected, as can be a possible fermion ball of hypothetical
heavy neutrinos. In fact all non-black hole configurations can be excluded by the available
measurements (Schödel et al 2003; Ghez et al 2005b) – except for a hypothetical boson
star and the gravastar hypothesis, but it appears that the two mentioned alternatives
have difficulties of their own, and they are less likely and certainly much less understood
than black holes (e.g. Maoz 1998; Miller et al 1998). We thus conclude that, under the
assumption of the validity of General Relativity, the Galactic Center provides the best
quantitative evidence for the actual existence of (massive) black holes that contemporary
astrophysics can offer.
5. Zooming in on the accretion zone and event horizon
Recent millimeter, infrared and X-ray observations have detected irregular, and
sometimes intense outbursts of emission from Sgr A* lasting anywhere between 30 min-
utes and a number of hours and occurring at least once per day (Baganoff et al 2001;
Genzel et al 2003a; Marrone et al 2006). These flares originate from within a few milli-
arcseconds of the radio position of Sgr A*. They probably occur when relativistic elec-
trons in the innermost accretion zone of the black hole are significantly accelerated, so
that they are able to produce infrared synchrotron emission and X-ray synchrotron or
inverse Compton radiation (Markoff et al 2001; Yuan et al 2003; Liu et al 2005). This
interpretation is also supported by the detection of significant polarization of the infrared
flares (Eckart et al 2006b), by the simultaneous occurrence of X- and IR-flaring activity
(Eckart et al 2006a; Yusef-Zadeh et al. 2006) and by variability in the infrared spectral
properties (Ghez et al 2005b; Gillessen et al 2006a; Krabbe et al. 2006). There are in-
dications for quasi-periodicities in the light curves of some of these flares, perhaps due
to orbital motion of hot gas spots near the last circular orbit around the event horizon
(Genzel et al 2003a; Aschenbach et al 2004; Bélanger et al 2006).
178 R. Genzel & V. Karas
Figure 3. Photo-center wobbling (left) and light curve (right) of a hot spot on the innermost
stable orbit around Schwarzschild black hole (inclination of 80 deg), as derived from ray-tracing
computations. Dotted curve: ‘true’ path of the hot spot; dashed curves: apparent path and a pre-
dicted light curve of the primary image; dash-dotted curves: the same for secondary image; solid
curves: path of centroid and integrated light curve. Axes on the left panel are in Schwarzschild
radii of a 3 million solar-mass black hole, roughly equal to the astrometric accuracy of 10 arcsec;
the abscissa axis of the right panel is in cycles. The loop in the centroids track is due to the
secondary image, which is strongly sensitive to the space-time curvature. The overall motion
can be detected at good significance at the anticipated accuracy of GRAVITY. Details can be
obtained by analyzing several flares simultaneously (Gillessen et al 2006b; Paumard et al 2005).
The infrared flares as well as the steady microwave emission from Sgr A* may be
important probes of the gas dynamics and space-time metric around the black hole
(Broderick & Loeb 2006; Meyer et al 2006a,b; Paumard et al 2006b). Future long-baseline
interferometry at short millimeter or sub-millimeter wavelengths may be able to map out
the strong light-bending effects around the photon orbit of the black hole. It is interesting
to realize that the angular size of the “shadow” of black hole (Bardeen 1973) is not very
far from the anticipated resolution of interferometric techniques and it may thus be
accessible to observations in near future (Falcke, Melia & Agol 2000).
Polarization measurements will help us to set further constraints on the emission
processes responsible for the flares. Especially the time-resolved lightcurves of the polar-
ized signal carry specific information about the interplay between the gravitational and
magnetic fields near Sgr A* horizon, because the propagation of the polarization vec-
tor is sensitive to the presence and properties of these fields along the light trajectories
(Bromley, Melia & Liu 2001; Horák & Karas 2006; Paumard et al 2006b). Polarization
is also very sensitive also to intrinsic properties of the source – its geometry and details
of radiation mechanisms responsible for the emission.
Synthesis of different techniques will be a promising way for the future: the as-
trometry of central stars gives very robust results because the stellar motion is almost
unaffected by poorly known processes of non-gravitational origin, while the flaring gas
occurs much closer to the black hole horizon and hence it directly probes the innermost
regions of Sgr A*. Eventually the two components – gas and stars of the Galaxy Center –
are interconnected and form the unique environment in which the flaring gas is influenced
by intense stellar winds whereas the long-term motion and the ‘non-standard’ evolution
of the central stars bears imprints of the gaseous medium though which the stars pass.
Eisenhauer et al (2005b) are developing GRAVITY (an instrument for ‘General
Relativity Analysis via VLT Interferometry’), which will provide dual-beam, 10 micro-
arcsecond precision infrared astrometric imaging of faint sources. GRAVITY may be able
to map out the motion on the sky of hot spots during flares with a high enough resolu-
The Galactic Center 179
tion and precision to determine the size of the emission region and possibly detect the
imprint of multiple gravitational images (see Fig. 3). In addition to studies of the flares,
it will also be able to image the orbits of stars very close to the black hole, which should
then exhibit the orbital radial oscillations and Lense-Thirring precession due to General
Relativity. Both the microwave shadows as well as the infrared hot spots are sensitive to
the space-time metric in the strong gravity regime. As such, these ambitious future ex-
periments can potentially test the validity of the black hole model near the event horizon
and perhaps even the validity of General Relativity in the strong field limit.
Acknowledgements
An extended version of this lecture was presented by RG as Invited Discourse during
the 26th General Assembly of the International Astronomical Union in Prague, 22nd
August 2006 (Highlights of Astronomy, Volume 14, 2007). VK thanks the Czech Science
Foundation for continued support (ref. 205/07/0052).
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	Introduction – Sagittarius A*
	High angular resolution astronomy
	Nuclear star cluster and the paradox of youth
	Compelling evidence for a central massive black hole
	Zooming in on the accretion zone and event horizon
