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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.
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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.
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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)
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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.
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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).
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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.
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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).
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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)
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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
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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
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Center”, 2000
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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
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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
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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
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