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Unveiling The Galactic Center (33) John W. Kulkosky, HET603B Group Member Project Supervisor: Yeshe Fenner Introduction What lies at heart of the Milky Way Galaxy? Many astronomers are convinced that the center of the Milky Way galaxy harbors a supermassive black hole. A black hole actually is a hole in the observable universe, a region of space where gravity has become so strong that nothing, not even light, can ever escape. The tremendous gravitational pull of a black hole originates from its immense mass and density. Astronomers know that a neutron star with a mass that is three times greater than the mass of the Sun cannot hold itself up under its own gravity and will compress into an infinitesimal point called a singularity, where the original matter is lost from view forever and only gravity remains, thus forming what is referred to as a stellar black hole. However, because evidence of the proposed supermassive black hole at the galaxy’s center suggests that it is indeed so massive, it could not have formed from the collapse of a single star. As a black hole consumes material, it becomes more massive so that the event horizon, the boundary where the escape velocity equals the speed of light, grows and the black hole occupies more space (Kormendy, 2000). Apparently, because of the crowded environment of the galactic center, an immense number of gas clouds and stars gradually merged together eventually forming a black hole of approximately 2.6 million solar masses and occupying a region that is a little more than 10% of the distance between the Sun and the Earth (approximately 15 million kilometers in diameter) (Zimmerman, 2001). Confirming that a black hole exists at the center of the Milky Way galaxy has proven to be quite challenging. Because black holes are dark objects, they cannot be observed directly. Furthermore, because of extinction by interstellar clouds of gas and dust, the galaxy’s core is hidden from our view at optical wavelengths. However, by using methods of indirect observation and telescopes sensitive to other wavelengths of the electromagnetic spectrum (radio, infrared, x-ray, and gamma ray), it has been possible for astronomers to study the structure and activity of the galaxy’s core and obtain mounting evidence of this black hole’s existence and location (Zimmerman, 2001). Components Of The Galaxy’s Center In the early 1930s, Karl Jansky, a researcher at Bell Telephone Labs in New Jersey, detected radio waves coming from the direction of the constellation Sagittarius (approximately 25,000 light years from Earth); the location Harlow Shapley concluded was the galactic center. Although the radio antenna’s resolution was far too crude to determine what he was observing, Jansky was convinced he had discovered a previously unknown interstellar object (Zimmerman, 2001). Today, images taken by such instruments as the Chandra X-ray Observatory and the Very Long Baseline Array have aided astronomers to construct a detailed portrait of the center of the Milky Way galaxy. 1 Astronomers recognize three principal components at the galaxy’s center: Sagittarius A East, Sagittarius A West, and the proposed supermassive black hole referred to as Sagittarius A* (denoted with an asterisk indicating its point source emission of radio waves) (see Figure 1). Figure 1 An x-ray image of the center of the Milky Way galaxy as observed by the Chandra X-ray Observatory. The large and small white dashed ellipses roughly represent the boundaries of Sagittarius A East and Sagittarius A West, respectively. [NASA/G. Garmire (PSU)/F. Baganoff (MIT)] Sagittarius A East – Surrounding and centered six light years from Sagittarius A* is a ring-shaped cloud referred to as Sagittarius A East. Using the orbiting Chandra X-ray Observatory, astronomers were able to separate Sagittarius A East from surrounding structures for the first time. The astronomers’ findings were able to support the longstanding hypothesis that Sagittarius A East is a supernova remnant that exploded about 10,000 years ago. "With Chandra, we found hot gas concentrated within the larger radio shell of Sagittarius A East," said Yoshitomo Maeda, an astronomy and astrophysics research associate at Penn State. "The gas is highly enriched by heavy elements, with four times more calcium and iron than the Sun, and that confirms earlier suspicions that Sagittarius A East is most likely a remnant of a supernova explosion” (Falcke, Melia and Agol; Chandra Press Room, 2001). It is believed that a supernova explosion creates a pair of shock waves, one traveling inward and one outward. The inward-moving shock wave heats material ejected from the explosion, while the expanding shock wave pushes gas comprising the interstellar medium outward. 2 According to Frederick Baganoff, a research associate at Massachusetts Institute of Technology and lead scientist for Chandra's Galactic Center project, "It is possible that the plowed gas has passed over the supermassive black hole at some time in the recent past. During the passage, a lot of gas could have been captured by the black hole." As matter is pulled in by the intense gravity of the black hole, it forms an accretion disk and is accelerated to nearly the speed of light releasing a large amount of energy (mainly in the form of x-rays) that can ionize the surrounding gas. Case in point, astronomers have detected a halo of ionized gas surrounding Sagittarius A East and Sagittarius A*, though x-ray emissions are weak, indicating the shock wave has indeed already passed the black hole (Chandra Press Room, 2001). The association between Sagittarius A East and Sagittarius A* may be an example of a common relationship between supernovae and supermassive black holes at the centers of galaxies. According to the Chandra scientists, individual supernova explosions may provide material that activates the accretion activity of these black holes on occasion, thereby allowing astronomers the ability to detect them (Chandra Press Room, 2001). Sagittarius A West – Sagittarius A West has been resolved into what astronomers have dubbed the Mini-spiral, a spiral-shaped cloud of ionized gas and dust, 10 to 20 light-years across. The Mini-spiral is divided into three components: the western, eastern and northern arms. It encompasses a region which includes Sagittarius A*, a cluster of hot luminous stars (IRS 16), a red supergiant star (IRS 7), a bubble region called the Minicavity, and a ring of molecular gas referred to as the Circumnuclear Disk (see Figure 2). Figure 2 An illustration of the features of Sagittarius A West, also referred to as the Mini-spiral. (American Scientist) 3 The Circumnuclear Disk spins around the galaxy’s core, in excess of 100 kilometers per second. Its gas is heated by the intense stellar winds and radiation created by the stars in IRS 16. These winds and radiation appear also to be responsible for stripping the surface off the red supergiant (IRS 7) to form an ionized comet-like tail and excavating a hole in the ionized gas, thus causing the formation of the Mini-cavity. In addition, astronomers have observed a stream of gas associated with the Circumnuclear Disk flowing past Sagittarius A* at speeds greater than 600 kilometers per second. The origin of this stream is still unknown. However, astronomers hypothesize that it may be the result of the breakup of a star several million years ago or a segment of the ring approaching too closely to the black hole and subsequently channeled into this stream by its intense gravity (American Scientist, 2000). Sagittarius A* – In 1974, Bruce Balick and Robert Brown discovered the most intense radio point source in the Milky Way galaxy. To mark its uniqueness, Brown named it Sagittarius A* (pronounced A-Star). Eventually, sufficient data was gathered to prove what astronomers long suspected, that Sagittarius A* coincided with the dynamical center of the galaxy. In other words, everything in the galaxy revolves around this radio source. This dynamical evidence (which will be covered in the next section of this paper) strongly indicates an extremely compact object. Furthermore, Sagittarius A* is very low in luminosity suggesting that it is most likely composed of dark matter. Therefore, the most plausible explanation is that Sagittarius A* is a black hole of considerable mass, what astronomers refer to as a supermassive black hole (Zimmerman, 2001). Evidence Supporting The Supermassive Black Hole Hypothesis By the 1980s, advancements in infrared telescopes allowed astronomers to observe individual stars within a light year of Sagittarius A*. These infrared observations showed that stars are tremendously crowded around the galaxy’s center, averaging about 1000 AU apart as compared to stars near the Sun separated by a distance of about 333,000 AU (Seeds, 1999). Furthermore, by tracking the motions of these stars around the center of the galaxy using a technique called “speckle imaging” to reduce the blurring effect of the Earth’s atmosphere, it was found that the closer the star is to Sagittarius A*, the faster it moved (Kormendy, 2000). Over the past ten years, astronomers have tracked the orbit of a particular star, dubbed S2, within 17 light-hours of Sagittarius A* (17 light-hours is about 3 times the distance from the Sun to Pluto). No other object has been observed so close to the heart of our galaxy. S2 has a very elongated orbit with a period of just 15.2 years. Its velocity exceeds 5000 kilometers per second – proof of the dark object’s extreme gravity, and therefore its extreme mass (Britt, 2002). The high velocities of the stars observed around Sagittarius A*, along with Newton’s form of Kepler’s third law, indicate a mass of about 2.6 million times the mass of the Sun at the center of the galaxy. Figure 3 is a graph of "enclosed mass", the amount of mass inside a particular distance from Sagittarius A*, versus distance from Sagittarius A*. The enclosed mass should decrease going towards the galaxy’s center because of the decrease in the number of stars. 4 Instead, the enclosed mass remains constant, indicating the mass in stars inside three light years cannot account for the orbital movement of gas and stars in the region. In addition, because of the implied high density, the theory of Sagittarius A* being a cluster of underluminous objects (brown dwarfs and dead stars) can be ruled out. This is also compelling evidence that the core of the galaxy is defined by a single, very massive black hole (Falcke, Melia and Agol; Kormendy, 2000). Supermassive Black Hole Underluminous Objects Luminous Stars Figure 3 A plot of distribution of enclosed mass versus distance from Sagittarius A*. The graph shows luminous stars, underluminous objects and the proposed single, supermassive black hole as possible mass models. (From Genzel and Eckart) Using the Very Long Baseline Array (VLBA), a series of ten radio telescopes located across the United States from St. Croix, the Virgin Islands to the Big Island of Hawaii, very accurate observations have been obtained to further support the existence of a supermassive black hole at the galaxy’s center. The VLBA uses a technique called interferometry, in which its ten telescopes are used in tandem to give it the resolution power of a single antenna the size of the Earth. The way Very Long Baseline Interferometry (VLBI) works is that each telescope’s signal is recorded on tape and then shipped to a central location where these signals are combined in what is referred to as the correlator. These combined recorded waves produce an interference pattern from which astronomers can reconstruct the structure of the observed sourced. From acquired VLBI data, Mark Reid of the Harvard-Smithsonian Center for Astrophysics has shown that Sagittarius A* has no proper motion. When observed in the sky, Sagittarius A* appears to move on a straight path, almost entirely within the plane of Milky Way – a result of the Sun orbiting the galactic center, not Sagittarius A* actually moving. Unlike nearby stars, which move at several hundred kilometers per second, Sagittarius A* remains virtually motionless, indicating that it is indeed an object of extreme mass at the center of the galaxy (Falcke, Melia and Agol; Gino, 2001). 5 VLBI data also indicates that Sagittarius A* is extremely compact. By utilizing higher frequencies (215 GHz), the VLBA was able to obtain higher resolution images, yielding a better measurement of Sagittarius A*’s size. Although the actual size of the source is still about 100 million kilometers, this technique has allowed astronomers to observe a region that is close to the actual black hole (Falcke, Melia and Agol). Some observations have also indicated the existence of high-energy jets emanating from the core of Milky Way galaxy. Such jets are characteristic of accretions disks around black holes. Furthermore, the orbiting Gamma Ray Observatory has detected gamma rays coming from the galaxy’s center and extending out into the halo. This gamma radiation is produced when an electron collides with its antiparticle, the positron. The origin of these positrons is uncertain, but they appear to be coming from high-energy processes associated with the core, possibly the hot accretion disk of a black hole (Seeds, 1999). Further evidence of the Milky Way galaxy harboring a supermassive black hole at its center is indirectly supported by data of the existence of black holes in the centers of other galaxies. In fact, many astronomers believe that most (if not all) large galaxies have massive black holes at their centers. A recent census has identified over 30 black holes within the cores of nearby galaxies, and more are expected to be found in distant galaxies in the near future using new telescopes such as the Space Infrared Telescope and the Next Generation Space Telescope. In addition, at least 10% of all known galaxies emit abnormally large amounts of energy from their centers, apparently powered by black holes accreting nearby gas and stars. These galaxies are referred to as active galaxies or AGN’s for active galactic nuclei (Kormendy, 2000). If large galaxies contain supermassive black holes in their cores, why aren’t all their cores as luminous as AGN’s? Some astronomers believe that these quiescent black holes may have run out of matter to consume, but others argue that most of the radiation generated in their vicinity gets pulled into the hole along with the infalling matter (Kormendy, 2000). Nevertheless, even inactive black holes can flare at times if a passing star or a cloud of dust or gas drifts too close. On October 26, 2000, while monitoring Sagittarius A* with the Chandra X-ray Observatory, Baganoff and colleagues recorded xray emissions 45 times higher than normal. However, within about three hours, the x-ray flare subsided and returned to previous levels. "The rapid rise and fall of x-rays from this outburst are compelling evidence that the x-ray emission is coming from matter falling into a supermassive black hole," explained Baganoff. "This is extremely exciting because it's the first time we have seen a supermassive black hole in our own neighborhood devour a chunk of material." Based on the amount of energy released, the Chandra team estimates that the consumed material had the mass of a comet (Chandra Press Room, 2001; Thomas, 2001). Observations indicate a correlation between masses of supermassive black holes and the size of their host galaxies. According to John Kormendy at the University of Texas, the mass of the black hole appears to be always about 0.2% of the mass of the galaxy's bulge. Therefore, the bigger the galactic bulge, the larger the black hole at the galaxy’s center. 6 It appears that supermassive black holes are connected with the formation and evolution of galaxies. Two theories suggest alternative explanations: (1) the black holes form first and then regulate galactic formation and evolution or (2) black holes and galaxies form and evolve together (Kormendy, 2000). Future Study Within the next few years, astronomers expect to detect the event horizon of the proposed black hole residing at the center of the Milky Way galaxy. According to Fulvio Melia, astrophysicist at the University of Arizona, by tracing the path of electromagnetic radiation through space warped by the gravity of the black hole, computer models are able to determine “the effects of the black hole on the radiation's path and wavelength, effects that are very precisely predicted by Einstein's Theory of General Relativity”. These computer models indicate a distinctive pattern in radiation from Sagittarius A*; this pattern is referred to as the black hole’s shadow (SpaceRef.com, 1999) (see Figure 4). Figure 4 One possible computer model of the shadow that might be cast by a black hole. The photons (light) that are not observed have vanished into the event horizon. (From Falcke, Melia and Agol). To actually detect the shadow of the supermassive black hole, astronomers have been using VLBI to observe shorter wavelengths of radio emission, a technique known as millimeter-VLBI. The highest resolution of current millimeter-VLBI is approximately 50 arcseconds, however, the supermassive black hole’s shadow is thought to have diameter of 30 arcseconds (Falcke, Melia and Agol, 2000). By extending the millimeter-VLBI to sub-millimeter wavelengths, the resolving power necessary to image the shadow will be possible. Currently, a number of telescopes and arrays are under construction or planned that will be capable of such resolution. Heino Flacke of the Max Planck Institute in Germany states, “This would be the final test of whether black holes and event horizons exist” (SpaceRef.com, 1999). 7 Einstein’s Theory of General Relativity also predicts that the motions of matter produce gravitational waves, distortions within the curvature of spacetime. Astronomers believe that detectable gravity waves are generated if the supermassive black hole at the galaxy’s center interacts with or consumes a sufficient amount of matter. These waves would manifest themselves by causing space and any objects in their path to expand and contract at right angles. However, these changes in dimension are expected to be much smaller than the diameter of an atomic nucleus. With gravitational wave detectors, such as the planned Laser Interferometric Space Array (LISA), astronomers will be able to detect these minute distortions, and therefore confirm the black hole’s existence (Visions 10). Determining The Distance To Our Galaxy’s Center Very Long Baseline Interferometry can also pinpoint the location of Sagittarius A* very accurately. With this acquired VLBI data, trigonometric parallax, the most direct and therefore the most reliable method to measure distance, can be utilized to determine the distance to center of the galaxy (Reed, 1999) (see Figure 5). In general, parallax is the apparent shifting of an object against the background, due to viewing it from different locations. In this case, the apparent shift in the position of Sagittarius A* is caused by observing it when the Earth is on opposite sides of its orbit relative to very distant quasars. From this measured difference in shift, along with basic trigonometric relations, the distance between the Sun and the galactic center can be calculated. Figure 5 A diagram showing how trigonometric parallax is used to determine the distance to the center of the galaxy. (From Reid) 8 The apparent shift in position of Sagittarius A* is a very small, about 0.1 milliarcseconds. In principle, VBLI can measure such a small shift. However, because of systematic sources of error, improvement of the VBLA’s current measurement accuracies is necessary. Reid is hopeful that within the next ten years the distance to the center of the Milky Way galaxy can be measured to very high accuracy with this technique (Reed, 1999). Samir Salim and Andrew Gould from Ohio State University are using an alternative method to determine the distance to the center of the galaxy. By analyzing the Keplerian orbits of individual stars in orbit around the black hole presumed to be at the galaxy’s center, they will be able to measure this distance to an accuracy of 4%, the most precise measurement to date (Gino, 2001). As the estimated distance to the center of our galaxy changes, so does the estimated distance to galactic and extra-galactic objects. An accurate measurement to the center of the galaxy (referred to as R0) is important for determining parameters, such mass and luminosity, of these objects. At present, the best estimate of R0 is 8.0 kiloparsecs (approximately 26,000 light years), with a standard error of about 0.5 kiloparsecs (Reed 1999; Gino, 2001). Conclusion Because of recent advancements in technology, there has been a dramatic increase in evidence to support the theory that the heart of the Milky Way galaxy is defined by a supermassive black hole. Indeed, it appears that this technological progress will confirm the existence of this black hole within the next several years. Astronomers believe that the study of the supermassive black hole will provide insight regarding our galaxy’s formation, structure and evolution. Furthermore, since supermassive black holes seem to be prevalent within the centers of many galaxies, their study will give a better understanding of the nature of galaxies as a whole. References American Scientist, “The Dark Monster”, August 2000, http://www.americanscientist.org/articles/00articles/meliaexcerpt2.html Britt, R. R., “Final Proof for Milky Ways’ Central Black Hole”, Space.com, October 2002, http://www.space.com/scienceastronomy/blackhole_milkyway_021016.html Chandra Press Room, “Scientists Discover Supernova May Control Activity in the Center of Our Galaxy”, February 2001, http://chandra.harvard.edu/press/01_releases/press_020101.html 9 Chandra Press Room, “Chandra Catches Milky Way Monster Snacking”, September 2001, http://chandra.harvard.edu/press/01_releases/press_090501flare.html Falcke, H., Melia, F. and Agol, E., “The Black Hole in the Galactic Center”, http://www.mpifr-bonn.mpg.de/staff/hfalcke/bh/sld1.html Falcke, H., Melia, F. and Agol, E., “The Shadow of the Black Hole at the Galactic Center”, 2000 Gino, M. C., “The Distance to the Galactic Center”, 2001 http://www.astrophys-assist.com/educate/distance/distance_gc.htm Kormendy, J. and Shields, G., “Monsters in Galactic Nuclei”, July 2000, http://chandra.as.utexas.edu/~kormendy/stardate.html Reid, M., “The Distance to the Center of the Milky Way”, 1999, http://cfa-www.harvard.edu/~reid/trigpar.html Reid, M., “Is Sag A* a Supermassive Black Hole?”, 1999, http://cfa-www.harvard.edu/~reid/sgra.html Sag A East Press Release, “Sagittarius A East Press Release Images”, January 2001, http://www.astro.psu.edu/users/maeda/GC/axaf/ Seeds M. A., Stars and Galaxies, Wadsworth Publishing Company, 1999 SpaceRef.com, “First Image of the Black Holes Shadow May Be Possible Soon”, December 1999, http://www.spaceref.com/news/viewpr.html?pid=327 Thomas, V., “Milky Way Black Hole Caught Snacking”, Astronomy.com, September 2001, http://www.astronomy.com/content/dynamic/articles/000/000/000/597dnnxn.asp Visions 10, “Gravity Waves”, http://physics.iop.org/Policy/v_production/v10.html Zimmerman, R., “Heart of Darkness”, Astronomy, October 2001 10