Marshall Space Flight Center
Chandra X-ray Observatory Center
Penn State University
University Park, PA
January 14, 2000
Frederick K. Baganoff and colleagues from Pennsylvania State University, University Park, and the University of California, Los Angeles, will present their findings today in Atlanta at the 195th national meeting of the American Astronomical Society.
Baganoff, lead scientist for the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer (ACIS) team's "Sagittarius A* and the Galactic Center" project and postdoctoral research associate at MIT, said that the precise positional coincidence between the new X-ray source and the radio position of a long-known source called Sagittarius A* "encourages us to believe that the two are the same."
Sagittarius A* is a point-like, variable radio source at the center of our galaxy. It looks like a faint quasar and is believed to be powered by gaseous matter falling into a supermassive black hole with 2.6 million times the mass of our sun.
Chandra's remarkable detection of this X-ray source has placed astronomers within a couple of years of a coveted prize: measuring the spectrum of energy produced by Sagittarius A* to determine in detail how the supermassive black hole that powers it works. "The race to be the first to detect X-rays from Sagittarius A* is one of the hottest and longest-running in all of X-ray astronomy," Baganoff said. "Theorists are eager to hear the results of our observation so they can test their ideas."
But now that an X-ray source close to Sagittarius A* has been found, it has taken researchers by surprise by being much fainter than expected. "There must be something unusual about the environment around this black hole that affects how it is fed and how the gravitational energy released from the infalling matter is converted into the X-ray light that we see," Baganoff said. "This new result provides fresh insight that will no doubt stir heated debates on these issues.
"Chandra's sensitivity is 20 times better than achieved with the best previous X-ray telescopes," said Gordon Garmire, the Evan Pugh Professor of Astronomy and Astrophysics at Penn State University and head of the team that conceived and built Chandra's Advanced CCD Imaging Spectrometer (ACIS) X-ray camera, which acquired the data. "This sensitivity, plus the superior spatial resolution of Chandra's mirrors, make Chandra the perfect tool for studying this faint X-ray source in its crowded field."
"The luminosity of the X-ray source we have discovered already is a factor of five fainter than previously thought, based on observations from an earlier X-ray satelllite," Baganoff said. "This poses a problem for theorists. The galactic center is a crowded place. If we were to find that most or all of the X-ray emission is not from Sagittarius A*, then we will have shown conclusively that all current models from Sagittarius A* need to be rethought from the ground up."
Astronomers believe that most galaxies harbor massive black holes at their centers. Many of these black holes are thought to produce powerful and brilliant point-like sources of light that astronomers call quasars and active galactic nuclei. Why the center of our galaxy is so dim is a long-standing puzzle.
More recently, infrared observations of the movements of stars around Sagittarius A* has convinced most astronomers that there is a supermassive black hole at the center of our galaxy and that it is probably associated with Sagittarius A*. A black hole is an object so compact that light itself cannot escape its gravitational pull. A black hole sucks up material thrown out by normal stars around it.
Because there are a million times more stars in a given volume in the galactic center than elsewhere in the galaxy, researchers cannot yet say definitively that Sagittarius A* is the newly detected source of the X-rays. "We need more data to clarify our observations," Baganoff said.
If Sagittarius A* is powered by a supermassive black hole, astronomers expected that there would be a lot of matter to suck up in a crowded place like the galactic center. The faintness of the source may indicate a dearth of matter floating toward the black hole or it may indicate that the environment of the black hole is for some reason rejecting most of the infalling material.
Only a few months after its launch, Chandra accomplished what no other optical or X-ray satellite was able to do: separate the emissions from the surrounding hot gas and nearby compact sources that prevented other satellites from detecting this new X-ray source. Mark Morris of the University of California at Los Angeles, who has studied this region intensely for 20 years, called Chandra's data "a gold mine" for astronomers.
"With more observing time on Chandra in the next two or three years, we will be able to build up a spectrum that will allow us to rule out various classes of objects and either confirm or deny Sagittarius A* as the origin of the X-ray emission," Baganoff said. "If we show that the emission is from a supermassive black hole, we will then be set to begin a detailed study of the X-ray emission from the nearest analog of a quasar or active galactic nucleus."
Chandra's ACIS detector, the Advanced CCD Imaging Spectrometer, was conceived and developed for NASA by Penn State University and MIT under the leadership of Penn State Professor Gordon Garmire.
To follow Chandra's progress or download images visit the Chandra
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program. TRW Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.
Chandra X-ray image of the innermost 10 light years at the center of our galaxy.
The image has been smoothed to bring out the X-ray emission from an extended cloud of hot gas surrounding the supermassive black-hole candidate Sagittarius A* (white dot at the center of the image). This gas glows in X-ray light because it has been heated to a temperature of millions of degrees by shock waves produced by supernova explosions and perhaps by colliding winds from young massive stars.
The study of objects in the universe using X-rays rather than visible light or other wavelengths of electromagnetic radiation. The X-rays can be imaged with grazing incidence mirrors which must be polished with extreme accuracy to reflect short-wavelength X-rays. An X-ray detector is placed at the focal plane of the telescope. The ACIS detector is a sophisticated version of the CCD detectors commonly used in video cameras or digital cameras.
CHANDRA X-RAY OBSERVATORY:
The latest in NASA's series of Great Observatories. Chandra is the "X-ray Hubble," launched in July 1999 on the Space Shuttle Columbia and then sent into a deep-space orbit around the Earth. Chandra carries a large X-ray telescope to focus the X-rays from objects in the sky. An X-ray telescope cannot work on the ground because the X-rays are absorbed by the Earth's atmosphere.
ACTIVE GALACTIC NUCLEUS:
The centers of some galaxies are unusually bright and variable in radio, infrared, optical, and X-ray light. These point-like sources are thought to be powered by gaseous matter falling into a supermassive black hole containing millions or billions of times the mass of our sun. Active galactic nucleus is a classification astronomers use to describe any bright, variable, and point-like source at the center of a nearby galaxy. Looking farther away, astronomers see sources which are even more luminous than the nuclei of nearby active galaxies. These sources are called quasars.
Harvard-Smithsonian Center for Astrophysics
January 12, 2000
Astronomers have long known that a supermassive black hole, more than 2 million times more massive than our Sun, lies at the center of the galaxy some 27,000 light-years from Earth. A point-like source of radio emission called Sagittarius A* (pronounced "A-star") marks the location of this black hole.
The black hole is surrounded by a ring of dust and gas orbiting Sagittarius A* (Sgr A*) at a radius of about 6.5 light-years from the black hole. This "circum-nuclear disk" revolves around the black hole at a velocity of 110 km/s. Gas and dust are stripped from the disk by the strong gravitational pull of the black hole and spiral towards Sgr A*.
The formation and dynamics of this circum-nuclear disk are unclear, so these astronomers have been looking for clues to the origin of the dust and gas seen in the circum-nuclear disk. Their most recent results come from the observation of ammonia molecules in the region around the disk. Emission from the ammonia molecules traces the dense, hot gas known to exist near the center of the galaxy and can be used to understand the location and motion of gas near the circum-nuclear disk.
Since the ammonia emission has a wavelength of 1.3 cm, the National Science Foundation's Very Large Array radio telescope at Socorro, NM, was used for the observations. The array is composed of 27 radio antennas, each 25 meters (82 ft) in diameter. For these observations, the antennas are arranged in a "Y" shape 3.4 km (2.1 mi) across. For observations at 1.3 cm, this allows the resolution of details as small as 0.13 light-years in diameter at the galactic center.
Narrow "streamers" of ammonia emission were observed connecting giant clouds of molecular gas to the circum-nuclear disk. These "giant molecular clouds" are located from 25 to 50 light-years from the center of the galaxy. The streamers are apparently pulled from the clouds by the strong gravitational force of the supermassive black hole. This finding agrees with earlier detections by Coil & Ho (1999) of ammonia emission at lower temperatures.
Because astronomers can only obtain 2-dimensional images, there could be a chance that the observed "streamer" is simply a free-floating cloud that only appears to be connected to the circum-nuclear disk. However, the velocity of the gas as well as the amount of turbulence in the gas suggests that the connection is real.
Emission from stationary ammonia molecules occurs at an exact frequency of 23.87 GHz. However, the frequency where it is picked up by the radio telescope can be shifted if the gas is moving. This shift is similar to the change in pitch of a train whistle as it passes by. Similarly, if the gas is moving in the direction of the Earth, then the emission is shifted to higher frequencies. If the gas is moving away, then it is shifted to lower frequencies.
The astronomers can use the frequency shift in the detected emission to study the velocity of the gas. Observations show that the velocities of gas increase as the streamers approach the circum-nuclear disk. A steady increase in the velocity of the gas as it moves inward indicates that gas is accelerating along the streamer towards the disk.
The frequency of the ammonia emission also allows the astronomers to study turbulence (amount of random motion) in the gas. This random motion means that the ammonia emission will be observed at a range of frequencies (corresponding to all of the different velocities present in the gas). The larger the range of frequencies, the more turbulence is present in the gas. The astronomers see that the range of frequencies increases as the streamers approach the circum-nuclear disk. As the gas approaches the disk, it responds to the stronger gravitational pull of the supermassive black hole. The pull of gravity affects the motions of the ammonia molecules and increases the turbulence in the gas. This observation also indicates that the streamers are approaching the circum-nuclear disk and are not a chance projection of another cloud.
These results imply that giant molecular clouds provide the circum-nuclear disk with gas and dust via "streamers" observable in ammonia emission. This dust and gas is then "fed" into the supermassive black hole. The detection of these streamers may answer many of the questions about the formation of the circum-nuclear disk and the interactions that take place at the center of the Milky Way.
More observations are necessary to fully understand the interaction of these "streamers" with the circum-nuclear disk. Observations of emission produced by cooler ammonia molecules will enable a direct calculation of the temperature of the gas. In addition, the astronomers hope to look for shocked gas in the region where the streamers appear to intersect the circum-nuclear disk. The presence of shocked gas in the region where the streamers intersect the disk would more strictly rule out the possibility of a chance projection of an intervening cloud onto the image.
This research was funded in part by the Harvard-Smithsonian Center for Astrophysics. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
A color image and caption of this result.
Ammonia emission (in yellow contours) in the region around the galactic center. The circum-nuclear disk as seen at a wavelength of 3 mm can be seen in the background color image (Gusten et al., 1987). Several streamers are seen to possibly be interacting with the CND and are labeled with numbers. The location of the supermassive black hole, Sgr A*, is labeled by a star.
University of California-Los Angeles
September 7, 1998
The question of what lies at the center of our galaxy, 24,000 light years away, has been the subject of a raging debate among astronomers for more than a quarter-century. Scientists have suspected that the galactic center contains either a single "supermassive" black hole or a cluster of millions of smaller stellar remnants. Black holes are collapsed stars so dense that nothing can escape their gravitational pull, not even light.
"Our galaxy is rather mild-mannered and quiet, and was one of the least likely galaxies to have a black hole at its center," said Ghez, an associate professor of physics and astronomy at UCLA, who spoke at The Central Parsecs: Galactic Center Workshop '98. "Yet a supermassive black hole at the center of our galaxy is precisely what we have found. The evidence for the black hole is very strong. One implication is that massive black holes may be found at the center of almost all galaxies."
The Milky Way is one of approximately 100 billion galaxies containing at least 100 billion stars each.
In her research, Ghez used the 10-meter Keck I Telescope -- the world's largest optical an infrared telescope -- atop Mauna Kea in Hawaii to study the movement of 200 stars that are close to the galactic center. Ghez studied these stars each year since 1995, using a technique she refined called "infrared speckle interferometry."
"Black holes cannot be seen directly, but their influence on nearby stars is very visible and provides a signature," said Ghez, 33. "We have found that signature in the rapid movement of the 20 or so stars that are most affected by its gravitational influence."
These 20 stars are orbiting ever closer to the black hole at a blinding speed of up to three million miles per hour -- about 10 times the speed at which stars typically move. The rapid speed at which the stars closest to the galactic center are moving reveals that the mass of the black hole -- 2.6 million solar masses -- must be concentrated in a single object, she said.
The star that was closest to the black hole in 1995 has since disappeared. Ghez has a number of possible theories to explain its disappearance, ranging from the mundane to the exotic. One explanation for observing a bright source in only one image, Ghez said, is that it was a "gravitational lensing event," which occurs when the light path from a star passing behind the black hole is bent by the strong gravitational field of the black hole. Alternatively, it could have been a flare due to a star falling into the black hole. Ghez, however, acknowledges that scientists may never learn which theory is correct.
One reason why astronomers previously had been unable to determine whether a black hole is at the galactic center is that our atmosphere distorts the images of stars. Ghez's speckle interferometry involves taking thousands of very quick, high- resolution snapshots that correct for the distortions produced by the Earth's atmosphere. She has developed algorithms - specific computer commands based on sophisticated mathematics -- and software for analyzing the data.
Using traditional imaging techniques at the center of the galaxy would cause the stars closest to the galactic center to look fuzzy and indecipherable. Ghez's technique, however, improves the resolution by a factor of at least 20.
"The atmosphere blurs your vision," Ghez said, "but speckle interferometry clears the picture up; it's like putting on glasses. Think of seeing a coin that looks distorted at the bottom of a pond. We take thousands of freeze frames, and then can determine what is distorted and what is really at the bottom of the pond."
The center of the Milky Way was identified in 1968 by Eric Becklin, a UCLA professor of physics and astronomy. Its general location in the galaxy is known, but not its precise location. The center of the Milky Way is located due south in the summer sky.
The black hole at the center of our galaxy came into existence billions of years ago, perhaps as very massive stars collapsed at the end of their life cycles and coalesced into a single, supermassive object.
Ghez studied the stars closest to the galactic center using the W.M. Keck Observatory's 10-meter Keck Telescope, and has returned to the Keck Observatory four times this year to observe the movement of these stars. She has been able to accurately predict the locations of the stars closest to the galactic center. (She identifies the stars based on their location and brightness.) Ghez has the highest resolution images of the galactic center ever obtained, which allow precise measurement of a group of stars close to the galactic center. Keck's large diameter allows Ghez to see fine details and to position the stars more accurately than a smaller telescope would permit -- details which were crucial in establishing the existence of the supermassive black hole.
"The Keck Observatory is the best facility in the world for this research," Ghez said. "The Keck Telescope enables us to track stars very precisely." The telescope's resolution is so high, Ghez added, that it could detect two flies in Japan that are less than 10 feet away from each other. "That's the resolution we are reaching," she said, "if you scale it out to 24,000 light years."
Ghez's research is supported by the National Science Foundation through an NSF Young Investigator Award, the Packard Foundation and the Alfred P. Sloan Foundation.
"Ghez's research is a real tour-de-force," said Ferdinand Coroniti, chair of UCLA's physics and astronomy department. "She continues to dazzle and amaze the astronomical community with her technical virtuosity and scientific accomplishments."
She is now searching for additional black holes or other dark matter near the massive black hole. Her research has been accepted for publication in the December issue of Astrophysical Journal. Ghez's co-authors on the paper are former UCLA graduate student Beth Klein and UCLA astronomy professors Mark Morris and Eric Becklin.
Harvard-Smithsonian Center for Astrophysics
These results, in conjunction with other work, were presented at the American Astronomical Society meeting here today by Mark Reid of the Harvard-Smithsonian Center for Astrophysics on behalf of an international team of astronomers including Anthony Readhead of Caltech, Rene Vermeulen from the Netherlands, and Robert Treuhaft of the Jet Propulsion Laboratory.
Because of its similarity to the active nuclei of other galaxies, astronomers have long suspected that Sgr A*, an extremely bright, point-like source of radio emission could be a massive black hole. However, the total power emitted by Sgr A* is comparatively low, less than emitted by some rare interacting stars. Thus, based on the strength of its emissions, Sgr A* does not have to be a very massive object.
Recently, the motions of stars very close to Sgr A* were measured by a group led by Andreas Eckart and Reinhard Genzel of Germany's Max-Planck-Institut fur Extraterrestrische Physik. They found extremely fast motions, some exceeding 1000 km/second, which would require a total mass nearly three million times that of the Sun centered at the position of Sgr A* and within a region of space only about 100 times larger than our Solar System.
What is the nature of this extraordinary mass concentration? Is Sgr A* a gigantic black hole, or simply an unusual group of stars? One way to determine this is to measure the motion of Sgr A* itself. If it is a massive black hole, it should stay anchored to the center of the Milky Way. On the other hand, if it is a single star (or small group of stars), then, like other stars in its vicinity, Sgr A* should be moving very rapidly.
The results presented at the AAS meeting include images of Sgr A* with the Very Long Baseline Array (VLBA), the National Science Foundation-supported array of radio telescopes which spans the USA from Hawaii to New England. These observations provided enough resolution to see Sgr A* move by many diameters in one year. After tracking Sgr A* for two years, Reid and his collaborators found that most of its apparent motion could be attributed to the Sun's orbit about the center of the Milky Way. (Although it takes over 200 million years for the Sun to completely circle the Milky Way, the effects of its orbit can be detected in only 10 days by VLBA observations!)
After correcting for solar effects, the remaining motion of Sgr A* is less than 20 km/sec, even slower than the Earth orbits the Sun. This result confirms similar measurements made with less intrinsic accuracy, but over a longer time period, using the Very Large Array by Don Backer of UC Berkeley and Dick Sramek of the National Radio Astronomy Observatory. Such a low speed rules out the option that Sgr A* is any single star, or even a small group of stars.
From the upper limit on the motion of Sgr A*, the astronomers conclude that its mass is certainly larger than a few thousand, and more likely some three million, times that of the Sun. The results are totally consistent with the theory that Sgr A* is a massive black hole anchoring the center of the Milky Way.
The speed of the ionized gas is so great that the tremendous gravitational pull of the dense galactic center, which is thought to be a black hole with a mass millions of times that of our sun, is unable to suck it in. Instead, it swings the ionized gas around it in a hyperbolic orbit and through a ring-shaped cloud of cool molecular gas that encircles the galactic center.
The findings represent 10 years of observations by a team led by Farhad Yusef-Zadeh, associate professor of physics and astronomy at Northwestern University, using the Very Large Array radio telescope in Socorro, N.M. He presented the results at the annual meeting of the American Astronomical Society.
Optical telescopes cannot see through the interstellar gas and dust within the disk-shaped galaxy, so scientists rely on radio telescopes and infrared telescopes to study the galactic center -- a maelstrom of awesome power that contains what Zadeh calls the galactic zoo of unusual and highly energetic phenomena -- 25,000 light years from Earth. The new finding adds yet another strange denizen to the zoo, but also may provide astronomers with a new set of clues to the nature of the mysterious black hole candidate.
"Stars basically follow the gravitational force, whereas this ionized gas follows a coherent flow that responds not only to gravitational pull but also to non-gravitational effects such as stellar winds, tidal forces and magnetic fields," Zadeh said.
Taken together with other observations, the flow may help clarify the three-dimensional picture of the whirlpool at the galactic center. Zadeh discussed his findings along with a German group that took infrared images of the motion of stars near the galactic center and a group from Harvard University that used radio telescope data to study the motion of the black hole.
Heated to more than ten thousand degrees, the gas stream exists as a plasma, its molecules stripped of their electrons to an ionized or electrically charged state. It hurtles around the galactic center at a closest distance of a fraction of a light year at more than two million miles per hour, Zadeh said.
"We dont know what its coming from, but something must have accelerated it to such high velocity," he said. "One thing thats obvious: this gas is not going to stick around for a long time. In a few thousand years, its outta here."
Zadeh's collaborators are Doug Roberts of the University of Illinois at Urbana-Champaign and John Biretta of the Space Telescope Science Institute in Baltimore.
CAPTION FOR GRAPHIC:
Drawing of swirling gas at the center of the Milky Way. The hot, ionized streamer is shown as a smooth swoosh swinging past the black hole at the galactic center; cooler molecular gas is dotted. Blue regions are moving toward Earth, red regions are moving away from us.