The Galactic Center

TL;DR

High-resolution infrared observations of the Galactic Center have confirmed a three million solar mass black hole, providing key evidence for black hole existence and insights into physics near the event horizon.

Contribution

This paper reviews recent high-resolution infrared measurements that establish the presence of a supermassive black hole at the Galactic Center and discusses implications for astrophysics and black hole physics.

Findings

Confirmation of a three million solar mass black hole

Detection of young massive stars near the black hole

Variable infrared and X-ray emissions from Sagittarius A*

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.

Figures

Full text

††volume: 238††journal: Black Holes: from Stars to Galaxies – across the Range of Masses††editors: V. Karas & G. Matt, eds.

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, Sagittarius 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 minutes (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 $\gamma$-ray emission within $10$ arcsec of Sgr A*. It is not yet clear whether these most energetic $\gamma$-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 $10^{5}$ 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. Investigations require measurements at longer wavelengths – the infrared and microwave bands, or at shorter wavelengths – hard X-rays and $\gamma$-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.

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 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 ${\propto}R^{-1.4\pm 0.1}$, 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 $\sim 10^{9}(R/(10^{\prime\prime}))^{-3}$ 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 $\mbox{K}\sim 16$ 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 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 $3.5$m 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 $\sim 0.1$ 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 $10$m diameter Keck telescope (Ghez et al. [1998]). Both groups independently found that the stellar velocities follow Kepler laws and exceed $10^{3}$ 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 $\sim 10$ milliarcsec. Most of the mass must be concentrated well within the peri-approaches of the innermost stars, $\sim 10$–$20$ light 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 $30$m+ diameter telescopes will be able to measure the mass and distance to the Galactic Center to $\sim 0.1$% 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 $2\sigma$ 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 $\sim 10^{5}$ 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 $10^{4}$ 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 minutes 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 electrons 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 indications 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]).

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 polarized signal carry specific information about the interplay between the gravitational and magnetic fields near Sgr A* horizon, because the propagation of the polarization vector 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 astrometry 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 resolution 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 experiments 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 $26^{\rm{}th}$ 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).