Outward Bound: From the Galactic Center to the Early Universe (englisch)


Reinhard Genzel

Balzan Preis 2003 für Infrarot-Astronomie

Professor Reinhard Genzel sind grundlegende Erkenntnisse im Bereich der Infrarot-Astronomie zu verdanken. Er entwickelte eine Methodik, die ihn und seine Kollegen in die Lage versetzte, aussergewöhnliche Entdeckungen zu machen. So wurde unter anderem die Existenz eines massiven Schwarzen Loches im Zentrum unserer Galaxie nachgewiesen.


Quasars and massive black holes

Quasars (‘quasi stellar radio sources’) are the most luminous objects in our Universe. Located at the nuclei of big galaxies these spectacular sources produce up to one hundred thousand times more electromagnetic energy than all of the tens of billions of stars in our Milky Way, albeit from within a volume of less than one light year across. These remarkable luminosities as well as the observed emission relativistic plasma jets and of very hard energy photons, at X- and g-rays, strongly suggest that the energy production is not nuclear fusion in stars. In contrast matter accretion onto massive black holes, with masses of a few million to a few billion times the mass of the Sun, can plausibly account for the quasar activity. This may appear at first paradoxical since the word ‘black’ reflects the basic prediction of General Relativity that black holes have a characteristic ‘event horizon’ from within which even photons cannot escape. When matter falls from the outside toward a black hole and is still outside its event horizon, however, gravitational energy can be converted with high efficiency into radiation. The ‘massive black hole’ paradigm is accepted by most astrophysicists. But a direct proof of the existence of a black hole requires the determination of its gravitational potential and, eventually, a proof of the existence of the event horizon.

The Center of the Milky Way
A direct experimental confirmation of whether a mass is in the extreme form of a black hole can be carried out by determining the motions of test particles at different distances from its center. If these motions are stable orbits around the central mass and if the orbital velocities scale like the inverse square root from the center, Kepler’s third law implies that there must be a central mass concentration. If one can observe test particles that are close enough to the center it is then in principle possible to conclusively test whether the configuration is that of a black hole. While simple in principle, this ‘Kepler experiment’ in practice cannot be executed in the very distant quasars. Even the largest telescopes currently do not have sufficient resolution to resolve their innermost regions. The situation is very different for the Center of our Milky Way. Although much less active than the luminous quasars, the Galactic Center is at a distance of a ‘mere’ 26,000 light years, one hundred times closer than the nearest external galaxies, and one hundred thousand times closer than the nearest quasars. In the Galactic Center the presently best measurements in the near-infrared resolve scales of a few light days, and intercontinental radio interferometry (VLBI) resolve scales of 10 light minutes. Interstellar dust particles along the line of sight between the Earth and the Milky Way Center strongly absorb visible radiation. Observations of the Galactic Center thus require measurements at longer wavelengths, in the infrared and radio bands, or measurements at very short wavelengths, at hard X-rays and g-rays, which can penetrate interstellar dust and gas.

The first evidence that the Galactic Center harbors a central mass concentration of a few million times the mass of the Sun emerged around 1978 from infrared spectroscopy of ionized gas clouds. These measurements were obtained by Nobel Laureate Charles Townes and his group at the University of California, Berkeley. I joined Prof.Townes’ group as a postdoctoral fellow in 1980. This marked the beginning of my fascination with the Galactic Center black hole problem. With novel measurements in the near-infrared (1-2.5mm) during the past decade my colleagues and I have been able to prove that this central mass indeed must be a massive black hole, beyond any reasonable doubt. As test particles for the gravitational potential we used stars. There are several hundred thousand stars concentrated in a dense cluster in the Milky Way Center.

Techniques of infrared astronomy
Due to the advent of sensitive semi-conductor detectors during the past two decades detection methods for near- and mid-infrared photons (wavelengths from 1 to 30mm) have become almost as efficient and powerful as in the visible part of the electromagnetic spectrum. Largely because of spin-off from military developments, we now have access to large format (>106 pixels) imaging detectors with very low electronic noise. In many ways these detectors are similar to the silicon-based CCDs that are the heart of commercial digital cameras. Because of the materials employed, however, infrared detectors can detect photons with wavelengths of up to a few tens of mm. At longer wavelengths, in the far-IR and submillimeter region (30 mm to 1mm), commercial detectors are not available and we had to develop our own detectors. Sensitive infrared instrumentation has to overcome one additional hurdle that is not present in the visible. The infrared sky is not dark at night, and every warm surface in the optical beam radiates a disturbing background of infrared photons (‘heat radiation’). Detectors and instruments have to be actively cooled to low temperatures, in some cases very close to the absolute zero point. Even the thermal background of telescope and atmosphere can be overcome by cooling the entire telescope structure and by placing the telescope in space, outside the perturbing atmosphere. Progress in infrared instrumentation, coupled with that in computing and analysis methods, has been enormous. During the past decade alone, improvements in sensitivity of a factor of several thousand have been realized. Over the last two decades we have built at MPE and UCB a number of instruments for infrared/submillimeter imaging and spectroscopy applications that have taken full advantage of these developments. We employed these instruments on large ground-based telescopes (near-infrared, submillimeter/mm wavelengths), or in aircraft and space platforms (mid- and far-infrared). The resulting improvements in our ability to investigate ever fainter (and more distant) sources in increasing detail have been most rewarding.

Another breakthrough has been the application of adaptive optics to large ground-based telescopes. As the wave front from a distant object passes through the turbulent Earth atmosphere it is distorted by refraction, resulting in image smearing. The effect is very much like that of the image of a distant car observed through the rising hot air above a summer road. As a result of this atmospheric seeing, a ten m-diameter telescope achieves no better images than a ten cm telescope. Adaptive optics overcomes this limitation by actively correcting for the atmospheric distortions. For this purpose, the wave front of a star close to the object under study is analyzed in the instrument and corrections are computed and instantaneously fed back to a deformable mirror in the optical train. The corrections re-sharpen the stellar and object images during the measurement. Modern adaptive optics systems achieve close to perfect, diffraction limited, performance in the infrared, and thus enhance image sharpness (angular resolution) by more than an order of magnitude. Unfortunately, it is relatively rare to find a bright enough star close to the object of interest. To overcome this shortcoming, it is possible to create an artificial star by focusing laser light into the upper atmosphere. If the laser is tuned to the 589nm D lines of atomic sodium, one can then exploit that there is a sodium layer at an altitude of about 90 km. The mesospheric sodium resonantly backscatters the laser light, appearing for the adaptive optics system as an artificial ‘star’. In collaboration between our Institute and the Max-Planck Institute for Astronomy (MPIA) in Heidelberg we were able in the mid-90s to successfully develop and employ Europe’s first, laser adaptive optics system for the 3.5m Calar Alto telescope. We are presently developing a more powerful version of this system for implementation at the 8m Very Large Telescope (VLT) of the European Southern Observatory (ESO).

A massive black hole in the Galactic Center
Employing several novel high-resolution cameras and imaging spectrometers at the ESO telescopes in La Silla (SHARP, FAST, 3D) during the 1990s, we were able to map out the properties and motions of stars at the center of the Milky Way, on scales from 0.3 to 30”, corresponding to linear scales of ten light days to several light years. One type of measurement involved taking repeatedly high resolution images over a number of years to determine the positional changes of stars on the sky (proper motions). By 1996, we were able to deduce from such measurements that the average stellar velocities indeed followed a ‘Kepler’ law, implying the presence of a mass concentration of three million solar masses associated with SgrA*, a compact radio source located at the very center of the nuclear star cluster. This mass concentration is dark (i.e. very little infrared radiation), thus has to be non-stellar and most likely is in form of a massive black hole. The most important breakthrough then came in 2002 and 2003 when we employed, partly in collaboration with three other groups in Germany and France, two new front-line instruments for the VLT, NACO and SPIFFI. The former is an adaptive optics camera, while the latter is a so-called integral field spectrometer that allows simultaneous wavelength-multiplexed spectroscopy over a contiguous two-dimensional field. During the initial test observation phases of NACO and taking advantage of the much improved image sharpness and sensitivity of the new instrument, we had the tremendous luck to track the motion of one star that approached SgrA* to within a mere 17 light hours. From the entire set of high resolution imaging data over the past decade we were then able to fully reconstruct the 15 year orbit of that star, S2, which turned out to be a perfect, highly eccentric Kepler ellipse. S2 orbits SgrA* just like the planets orbit the Sun. In fact the orbital period of S2 is similar to that of Jupiter and its peri-center distance to SgrA* is only three times greater than the orbit of the planet Pluto. These data and measurements obtained independently by a second group working with the 10m Keck telescope in Hawaii, clearly demonstrate that all previously proposed, stable configurations of matter other than a black hole can now be excluded. Additional measurements obtained in spring and summer 2003 resulted in the discovery of infrared flares from the black hole. The rapid variability of the flares indicates that the infrared emission comes from just outside the event horizon. Their time structure suggests that the central black hole may be spinning rapidly. The Galactic Center now represents the best case we have from astronomical measurements for the existence of massive black holes postulated 40 years ago. The Galactic Center has become a unique laboratory for studying the space time structure and environment around such a black hole.

Black holes and galaxies in the early Universe
Much evidence has accumulated during the past decade that massive black holes are common also in the nuclei of nearby, external galaxies. The measurements in these more distant galaxies generally cannot prove unambiguously that the central mass must be in form of a black hole. The analogy to the Galactic Center makes that conclusion nevertheless very probable. There is also one external galaxy in which intercontinental radio interferometry is able to determine the rotation of a small gas disk very close to the central mass and thus provides very strong evidence that the object must be a black hole. Essentially all large external galaxies appear to harbor a central dark mass at their nucleus. Black hole mass appears to scale with the mass of the galaxy itself. About one tenth of a percent of the mass of a galaxy appears to be concentrated in its central black hole. In some cases the central black holes has a mass of several billion solar masses, about a thousand times that in the Milky Way. The question then is how this relationship came about and when and how the most massive black holes were formed.

While the Galactic Center black hole appears to be growing slowly over time from low level accretion of gas from its neighborhood, the growth of the most massive black holes requires more violent processes. The recent discovery of luminous quasars at cosmological redshifts as high as 6 suggests that many of the most massive black holes were already in place about 1 to 2 billion years after the Big Bang. At that time the Universe also went through its most active phase of star formation and large galaxies were assembled from mergers of smaller subunits. Galaxy formation and black hole formation appear to be intimately related. We and other groups have recently been able to show that very massive, dust and gas rich galaxies formed remarkably rapidly at high redshift and that such massive host galaxies often harbor luminous quasars. The billion solar mass black holes found in nearby elliptical galaxies thus may be the ashes of these earlier, luminous high redshift systems.

It appears that the ‘collision’ and complete merging of two galaxies is a key process in the formation and evolution process of galaxies and massive black holes. While such mergers happen much more rarely in the local Universe than in the dense young Universe, they still do and there we can study the physical processes in much greater detail. In 1996/1997 we were able to make a set of key measurements with a new, very sensitive spectrometer for mid-infrared wavelengths that a group of Dutch scientists and we had developed for the Infrared Space Observatory (ISO), a small telescope cooled to a few degrees above absolute zero and employed by the European Space Agency (ESA) in a 24 hour orbit around the Earth. With ISO we could, for the first time, study the infrared spectra of a class of distant, very luminous mergers. The infrared observations let us penetrate their veils of dust and allowed us to analyze the sources of luminosity deeply hidden at the centers of the mergers. We found that in the last stages of a galaxy-galaxy merger both intense star formation and accretion onto the central black holes contribute to the total luminosity, although star formation appears to overall dominate in most cases. In accordance with the above discussion, the major star formation events and efficient fueling of the central black holes are simultaneously triggered by the cosmic ‘traffic accidents’.

A career in experimental astrophysics
I have discussed above some of the research that my students, postdocs, colleagues and I have been engaged in during the past decade. It has been an exciting time. It is fantastic that we could play an active role in the discoveries in the present ‘golden era’ of astronomy and astrophysics. In a mere two decades astrophysicists have been able to determine the basic large scale structure and evolution of the Universe, began to understand how galaxies and black holes formed and evolved, and detected extrasolar planets. A key element in this rapid evolution has been the rapid progress in telescopes, instruments, detectors and computers, which have driven the rate and scope of the discoveries. Infrared astronomy is a case in point. How did I end up in this field?

Originally I wanted to become an archaeologist. Then my father, a solid state physicist and gifted experimentalist, excited my interest and curiosity in science and taught me physics. He was a wonderful teacher. Starting with the usual chemistry experiments at home (whose poisonous and acidic results destroyed carpets and wall paper), I matured after a few years to building my first spectrometer. It featured a wonderful large prism, a collimator/camera and a high-voltage spark as spectral light source. The triumph was to spectrally resolve the two Na-D lines mentioned already above.

After undergraduate studies in physics at the University of Freiburg, I moved to Bonn University to study astrophysics and radio astronomy. Again my father’s advice was a key element for this decision. A new Max-Planck Institute had just been founded there, and the 100 m Effelsberg radio telescope had been built, at that time the world’s largest fully steerable radio telescope. I was accepted as a master student in the group of Peter Mezger. I worked under the excellent and collegial supervision of Arno Witzel on the analysis of the radio spectral energy distributions of distant radio galaxies and quasars. I then got excited about radio spectroscopy of interstellar molecules. Molecules in interstellar space had been discovered a few years before and more and more complex species were being detected. Their origin and possible relationship to life was a mystery. I thus changed to Peter Mezger’s molecular spectroscopy group, and a very productive and long lasting collaboration started with Dennis Downes who supervised my Ph.D. thesis. We began investigating interstellar water vapor masers that had previously been shown to be small, very dense interstellar cloudlets that amplify by stimulated emission the radiation of a 1.3cm transition of the molecule. Working with the new 100m telescope we mapped out the sites of maser emission in the Galaxy. We demonstrated that the masers were associated with the last stages of the formation of massive stars. Combining the 100m with other large radio telescopes to an intercontinental radio interferometer we started a series of experiments to unravel the small scale structure and dynamics of the maser cloudlets. The culmination of these experiments came a few years later, when I already was a postdoctoral fellow with James Moran at the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Massachusetts. We found from the precise radio measurements that the maser cloudlets moved in space and that these motions could be ascribed to mass outflow in the star forming regions. These and other observations established that star formation and substantial gas outflow are intimately connected. For a gas cloud to collapse to form a new star most of its original angular momentum has to be removed. The mass outflows, probably mediated by magnetic fields, are the vehicle to remove the angular momentum

In 1980 I was offered the fantastic opportunity, through Peter Mezger, to join Charles Townes’ group at the University of California, Berkeley. Charles Townes, 1964 Nobel Laureate for the development of the maser and the laser, had moved to Berkeley in the late 1960s and started a world famous effort in infrared astronomy. The move to Berkeley, initially as a postdoc and then as a young member of the physics faculty marked the beginning of a very productive and exciting time and of my work in experimental infrared astronomy. Charles Townes became my second father and scientific mentor. In the 1980s we developed instruments to carry out the first detailed spectroscopy of dense interstellar gas in the 50 to 500mm region, in part on the NASA Kuiper Airborne Observatory and in part from telescopes on the 4300m Mauna Kea in Hawaii. Water vapor absorbs far-infrared and submillimeter waves and it is necessary to go to dry, high mountain tops, airplanes or space to make astronomical measurements. These measurements gave valuable new information on the interaction of massive stars with their interstellar and galactic environment and on the structure of dense molecular clouds.

In 1985 I was approached by Gerhard Haerendel from the Munich based Max-Planck Institute for Extraterrestrial Physics (MPE) with the proposal to join the Institute and head their infrared astronomy effort. MPE had been founded by Reimar Lüst in 1963 and by 1986 had become a major European center in space astrophysics. Although parting from California and the exciting atmosphere in the UCB physics and astronomy departments came not easy, I returned to Germany in 1986 and soon started some of the experimental and scientific projects that I have described in more detail above. I have recently followed again the lure of California and accepted a part-time appointment as professor in the UCB physics department. In all of this I have been lucky that my family has been willing to follow me all over the world and tolerate and support my intense life in astrophysical research.

The infrared part of the electromagnetic spectrum will almost certainly continue to play a key role in future discoveries and key developments. The next step beyond adaptive optics with large telescopes will be their interferometric combination, resulting in another order of magnitude increase in angular resolution. This leap will come in the next few years and will be first realized at infrared wavelengths. In the Galactic Center we hope to use these interferometric techniques to explore the relativistic regime on a scale of a few light hours around the black hole. Infrared interferometry will also likely lead to the first direct detection and imaging of extrasolar planets. With increasing cosmological redshift the optical and ultraviolet radiation from galaxies and quasars in the early Universe is shifted into the infrared. In addition, the most luminous high redshift galaxies appear to be dust enshrouded, requiring observations at far-infrared and submillimeter wavelengths. Ground-based, airborne and space infrared/submillimeter telescopes will thus be at center stage for gaining a detailed understanding how the first galaxies and black holes formed. The future thus appears bright. I hope that my colleagues and I can continue to play an active role in this adventure. The support of the Balzan Prize will be an important foundation and stimulus for our work.

