ASTRONOMY: NEW IMAGES OF MILKY WAY BLACK HOLE
ScienceWeek http://scienceweek.com
The following points are made by Christopher Reynolds (Nature
2005 438:32):

1) Ever since its discovery in 1974, a strange source of radio
waves in the constellation of Sagittarius has been suspected of
flagging the presence of a massive black hole at the center of
our Galaxy[1]. New work[2] reports the highest-resolution images
yet of this source, Sgr A*. These observations provide strong
evidence that Sgr A* is indeed a black hole, and afford a glimpse
of the behavior of the matter that is about to flow into it. They
are also a further step towards attaining an image of the shadow
around the edge of a black hole, a powerful and classic test of
the general theory of relativity.

2) Black holes are perhaps the most exotic objects to impinge on
the cosmic consciousness. They are formed when matter such as
that from a dying massive star collapses inward calamitously
under its own gravity, forming a region of space in which the
gravitational field is so strong that it swallows all matter and
radiation that come near it. Delineating this region is the
"event horizon", the point of no return beyond which no matter or
light can ever escape.

3) Sgr A* is certainly an exotic object. Through observations at
infrared frequencies of the bright stars speeding around it,
astronomers have confirmed that it is four million times more
massive than our Sun and confined to a region of space at the
exact center of the Galaxy[3,4] that is no bigger than the region
enclosed by the orbit of Pluto. Such a high concentration of mass
puts tight constraints on the possible nature of the object. A
cluster of several million neutron stars, themselves collapsed
dead stars, could be as heavy as that, but could only survive in
such a compact form for 20,000 years or so -- a blink of an eye
in astronomical terms -- before either collapsing further (to a
black hole) or evaporating away into space. It is unlikely that
we are observing the Galactic center just when such a bizarre
neutron-star cluster happens to exist. There is only one other
possibility, however, if standard physics -- the standard model
of particle physics, coupled with the general theory of
relativity -- is to hold. That is that Sgr A* harbors a
supermassive black hole.

4) Shen et al[1] use a technique known as Very Long Baseline
Interferometry (VLBI), which correlates information from radio
antennae at separate remote locations, thereby increasing the
spatial resolution for images of far-off objects. The authors
used the Very Long Baseline Array, a system of ten radio
telescopes scattered across the United States with a maximum
separation of some 8000 kilometers. They built up a picture of
the radio emission at a wavelength of 3.5 centimeters from gas in
a region just 8 light minutes across, centered on the putative
black hole. Even with the most conservative assumptions, the
authors find lower limits on the concentration of mass that are a
factor of 100,000 higher than those derived from the motions of
the stars surrounding it. This would reduce the lifetime of any
neutron-star cluster there to a mere 100 years, a result that
must dispel any lingering notions that the source at Sgr A* is a
compact cluster of known objects.

5) But we should guard against complacency: nature might have
some surprises in store. Could it be that standard physics is
inadequate and that, other than a black hole, there are stable
objects that have the compact, huge mass of Sgr A*? What is
needed is a more discerning test than simply detecting something
massive and compact: we need to find the event horizon, the
defining property of a black hole.

References (abridged):

1. Balick, B. & Brown, R. L. Astrophys. J. 194, 265-270 (1974)

2. Shen, Z. -Q., Lo, K. Y., Liang, M. -C., Ho, P. T. P. & Zhao,
J-H. Nature 438, 62-64 (2005)

3. Ghez, A. M. et al. Astrophys. J. 620, 744-757 (2005)

4. Schoedel, R. et al. Nature 419, 694-696 (2002)

5. Garcia, M. et al. Astrophys. J. 553, L47-L50 (2001)

Nature http://www.nature.com/nature

--------------------------------

Related Material:

ASTRONOMY: ON THE CENTER OF THE MILKY WAY

The following points are made by T.J. Lazio and T.N. LaRosa
(Science 2005 307:686):

1) At a distance of just 25,000 light years (2.5 x 10^(20) m),
the center of our Galaxy, the Milky Way, provides the foundation
for understanding phenomena in other galaxies. The central black
hole (1) and regions of intense star formation in its vicinity
can be probed at 100 times the resolution of even the nearest
galaxies. Nonetheless, even the basic properties of a key
component of the galactic center, its magnetic field, remain
poorly understood.

2) Magnetic fields have the potential to transform, store, and
explosively release energy, to transport angular momentum, and to
confine high-energy plasmas into powerful jet flows. They are
therefore central to astrophysical activity from stellar to
galactic scales.

3) Magnetic fields are found throughout the Milky Way.
Measurements suggest that the magnetic field in the spiral disk
of our Galaxy has two components, one globally ordered and the
other random, with approximately equal strengths of ~0.3 nT (2);
the globally ordered component generally follows the spiral arms
of the galaxy. Key questions about the magnetic field in the
galactic center are whether it is comparable in strength or much
stronger than the field in the disk, and whether it is globally
ordered or largely random.

4) approximately 20 years ago, the first high-resolution radio
images of the galactic center (3) revealed numerous magnetic
structures that are unique to the galactic center. The most
striking of these is the galactic center radio arc, a series of
parallel linear filaments, each of which is merely a few light
years wide yet more than 100 light years long. Also observed were
a number of isolated linear features that were variously referred
to as streaks, threads, and filaments. The relation between these
isolated filaments and the bundled filaments of the radio arc
remains unknown.

5) These filamentary structures are distinguished by extreme
length-to-width ratios (~10 to 100), nonthermal spectra, and a
high intrinsic polarization (~30%, and in some cases approaching
the theoretical maximum of 70% for synchrotron radiation). The
polarization and nonthermal spectra are consistent with the
filaments being produced by synchrotron radiation from
relativistic electrons spiraling around a magnetic field.
Detailed measurements of individual filaments have shown that the
magnetic fields are aligned longitudinally with the
filament.(4,5)

References (abridged):

1. G. C. Bower et al., Science 304, [704] (2004)

2. R. Beck, Space Sci. Rev. 99, 243 (2001)

3. F. Yusef-Zadeh et al., Nature 310, 557 (1984)

4. M. Morris, E. Serabyn, Annu. Rev. Astron. Astrophys. 34, 645
(1996)

5. C. C. Lang, K. R. Anantharamaiah, N. E. Kassim, T. J. W.
Lazio, Astrophys. J. 521, L41 (1999)

Science http://www.sciencemag.org

--------------------------------

Related Material:

ON THE BLACK HOLE AT THE CENTER OF OUR GALAXY

The following points are made by Alexei V. Filippenko (Proc. Nat.
Acad. Sci. 1999 96:9993):

1) Some galaxies are known to have very "active" central regions
from which enormous amounts of energy are emitted each second.
These "active galactic nuclei" are probably powered by accretion
of matter into a supermassive black hole of 10^(6) to 10^(9)
solar-masses. The center of our own Galaxy exhibits mild
activity, especially at radio wavelengths: so-called "nonthermal
radiation" characteristic of high-energy electrons spiraling in
magnetic fields is emitted by a compact object at the Galactic
center known as *Sagittarius A*. Does the center harbor a
supermassive black hole?

2) One approach is to determine whether stars in the central
region are moving very rapidly, as would be expected if a large
mass were present. During the past 5 years, two teams have
obtained high-resolution images of our Galactic center, each team
on several occasions, so that temporal changes in the positions
of stars could be detected. The observations were conducted at
infrared wavelengths, which penetrate the gas and dust between
Earth and the Galactic center (a distance of approximately 25,000
light years) much more readily than optical light. In summary,
the data are in excellent agreement with the conclusion that the
gravitational potential of the central region of our Galaxy is
dominated by a single object. The derived mass of this object is
(2.6 +- 0.2) x 10^(6) solar-masses, and the mass density within a
radius of 0.05 light-years is at least 6 x 10^(9) solar-masses
per cubic light-year, effectively eliminating all possibilities
other than a black hole.

3) Although our Galaxy provides the most convincing case for the
existence of supermassive black holes, observations of the
centers of a few other galaxies bolster the conclusion. For
example, very precise measurements of the galaxy NGC 4258 reveal
a central compact object with a derived mass 3.6 x 10^(7) solar-
masses. On somewhat larger scales, spectra obtained with the
Hubble Space Telescope show gas and stars rapidly moving in a
manner consistent with the presence of a supermassive black hole.
The most massive existing case, that of the giant elliptical
galaxy M87, is approximately 3 x 10^(9) solar-masses. Moreover,
x-ray observations of some active galactic nuclei reveal emission
from a hot disk of gas apparently very close to a black hole,
since extreme relativistic effects are detected. In general, it
now seems that a supermassive black hole is found in nearly every
large galaxy amenable to such observations.

4) The author concludes: "In the last decade of the 20th century,
black holes have moved firmly from the arena of science fiction
to that of science fact. Their existence in some *binary star
systems, and at the centers of massive galaxies, is nearly
irrefutable. They provide marvelous laboratories in which the
strong-field predictions of Einstein's general theory of
relativity can be tested."

Proc. Nat. Acad. Sci. http://www.pnas.org

--------------------------------

Notes by ScienceWeek:

Sagittarius A*: Sagittarius A is a prominent radio source in the
constellation Sagittarius, coincident with or close to the center
of our Galaxy. It is a highly complex region consisting of a
central core approximately 50 light-years in diameter.
Sagittarius A* is a compact component at the heart of the central
core of Sagittarius A. Sagittarius A* is an intense source of
radio waves, and is apparently unique in our Galaxy: while
everything else in our Galaxy is on the move as they follow their
orbits, Sagittarius A* is absolutely stationary and must
therefore lie exactly at the Galaxy's center. Many astronomers,
in fact, use Sagittarius A* as the "Greenwich Meridian" of the
Galaxy.

binary star systems: Binary stars are a pair of stars revolving
around a common center of mass under the influence of their
mutual gravitational attraction, and apparently the majority of
stars in the Universe are binaries and not singlets. In some
cases the binary system is resolvable into two components, and in
other cases the presence of a second star is inferred by
perturbations in the motion or emitted radiation of the first
star. If the binaries are close enough, they may share stellar
material, and this results in a particular kind of stellar
evolution. In the black hole-binary systems mentioned in this
report, matter transfers from a relatively normal star (known as
the "secondary star") to a dark compact object (the "primary").
Recent comparisons of x-ray and optical brightness suggest that
in many cases the dark primary in such a binary system is a black
hole.

ScienceWeek http://scienceweek.com

ASTRONOMY: NEW IMAGES OF MILKY WAY BLACK HOLE
ScienceWeek http://scienceweek.com
The following points are made by Christopher Reynolds (Nature
2005 438:32):

1) Ever since its discovery in 1974, a strange source of radio
waves in the constellation of Sagittarius has been suspected of
flagging the presence of a massive black hole at the center of
our Galaxy[1]. New work[2] reports the highest-resolution images
yet of this source, Sgr A*. These observations provide strong
evidence that Sgr A* is indeed a black hole, and afford a glimpse
of the behavior of the matter that is about to flow into it. They
are also a further step towards attaining an image of the shadow
around the edge of a black hole, a powerful and classic test of
the general theory of relativity.

2) Black holes are perhaps the most exotic objects to impinge on
the cosmic consciousness. They are formed when matter such as
that from a dying massive star collapses inward calamitously
under its own gravity, forming a region of space in which the
gravitational field is so strong that it swallows all matter and
radiation that come near it. Delineating this region is the
"event horizon", the point of no return beyond which no matter or
light can ever escape.

3) Sgr A* is certainly an exotic object. Through observations at
infrared frequencies of the bright stars speeding around it,
astronomers have confirmed that it is four million times more
massive than our Sun and confined to a region of space at the
exact center of the Galaxy[3,4] that is no bigger than the region
enclosed by the orbit of Pluto. Such a high concentration of mass
puts tight constraints on the possible nature of the object. A
cluster of several million neutron stars, themselves collapsed
dead stars, could be as heavy as that, but could only survive in
such a compact form for 20,000 years or so -- a blink of an eye
in astronomical terms -- before either collapsing further (to a
black hole) or evaporating away into space. It is unlikely that
we are observing the Galactic center just when such a bizarre
neutron-star cluster happens to exist. There is only one other
possibility, however, if standard physics -- the standard model
of particle physics, coupled with the general theory of
relativity -- is to hold. That is that Sgr A* harbors a
supermassive black hole.

4) Shen et al[1] use a technique known as Very Long Baseline
Interferometry (VLBI), which correlates information from radio
antennae at separate remote locations, thereby increasing the
spatial resolution for images of far-off objects. The authors
used the Very Long Baseline Array, a system of ten radio
telescopes scattered across the United States with a maximum
separation of some 8000 kilometers. They built up a picture of
the radio emission at a wavelength of 3.5 centimeters from gas in
a region just 8 light minutes across, centered on the putative
black hole. Even with the most conservative assumptions, the
authors find lower limits on the concentration of mass that are a
factor of 100,000 higher than those derived from the motions of
the stars surrounding it. This would reduce the lifetime of any
neutron-star cluster there to a mere 100 years, a result that
must dispel any lingering notions that the source at Sgr A* is a
compact cluster of known objects.

5) But we should guard against complacency: nature might have
some surprises in store. Could it be that standard physics is
inadequate and that, other than a black hole, there are stable
objects that have the compact, huge mass of Sgr A*? What is
needed is a more discerning test than simply detecting something
massive and compact: we need to find the event horizon, the
defining property of a black hole.

References (abridged):

1. Balick, B. & Brown, R. L. Astrophys. J. 194, 265-270 (1974)

2. Shen, Z. -Q., Lo, K. Y., Liang, M. -C., Ho, P. T. P. & Zhao,
J-H. Nature 438, 62-64 (2005)

3. Ghez, A. M. et al. Astrophys. J. 620, 744-757 (2005)

4. Schoedel, R. et al. Nature 419, 694-696 (2002)

5. Garcia, M. et al. Astrophys. J. 553, L47-L50 (2001)

Nature http://www.nature.com/nature

--------------------------------

Related Material:

ASTRONOMY: ON THE CENTER OF THE MILKY WAY

The following points are made by T.J. Lazio and T.N. LaRosa
(Science 2005 307:686):

1) At a distance of just 25,000 light years (2.5 x 10^(20) m),
the center of our Galaxy, the Milky Way, provides the foundation
for understanding phenomena in other galaxies. The central black
hole (1) and regions of intense star formation in its vicinity
can be probed at 100 times the resolution of even the nearest
galaxies. Nonetheless, even the basic properties of a key
component of the galactic center, its magnetic field, remain
poorly understood.

2) Magnetic fields have the potential to transform, store, and
explosively release energy, to transport angular momentum, and to
confine high-energy plasmas into powerful jet flows. They are
therefore central to astrophysical activity from stellar to
galactic scales.

3) Magnetic fields are found throughout the Milky Way.
Measurements suggest that the magnetic field in the spiral disk
of our Galaxy has two components, one globally ordered and the
other random, with approximately equal strengths of ~0.3 nT (2);
the globally ordered component generally follows the spiral arms
of the galaxy. Key questions about the magnetic field in the
galactic center are whether it is comparable in strength or much
stronger than the field in the disk, and whether it is globally
ordered or largely random.

4) approximately 20 years ago, the first high-resolution radio
images of the galactic center (3) revealed numerous magnetic
structures that are unique to the galactic center. The most
striking of these is the galactic center radio arc, a series of
parallel linear filaments, each of which is merely a few light
years wide yet more than 100 light years long. Also observed were
a number of isolated linear features that were variously referred
to as streaks, threads, and filaments. The relation between these
isolated filaments and the bundled filaments of the radio arc
remains unknown.

5) These filamentary structures are distinguished by extreme
length-to-width ratios (~10 to 100), nonthermal spectra, and a
high intrinsic polarization (~30%, and in some cases approaching
the theoretical maximum of 70% for synchrotron radiation). The
polarization and nonthermal spectra are consistent with the
filaments being produced by synchrotron radiation from
relativistic electrons spiraling around a magnetic field.
Detailed measurements of individual filaments have shown that the
magnetic fields are aligned longitudinally with the
filament.(4,5)

References (abridged):

1. G. C. Bower et al., Science 304, [704] (2004)

2. R. Beck, Space Sci. Rev. 99, 243 (2001)

3. F. Yusef-Zadeh et al., Nature 310, 557 (1984)

4. M. Morris, E. Serabyn, Annu. Rev. Astron. Astrophys. 34, 645
(1996)

5. C. C. Lang, K. R. Anantharamaiah, N. E. Kassim, T. J. W.
Lazio, Astrophys. J. 521, L41 (1999)

Science http://www.sciencemag.org

--------------------------------

Related Material:

ON THE BLACK HOLE AT THE CENTER OF OUR GALAXY

The following points are made by Alexei V. Filippenko (Proc. Nat.
Acad. Sci. 1999 96:9993):

1) Some galaxies are known to have very "active" central regions
from which enormous amounts of energy are emitted each second.
These "active galactic nuclei" are probably powered by accretion
of matter into a supermassive black hole of 10^(6) to 10^(9)
solar-masses. The center of our own Galaxy exhibits mild
activity, especially at radio wavelengths: so-called "nonthermal
radiation" characteristic of high-energy electrons spiraling in
magnetic fields is emitted by a compact object at the Galactic
center known as *Sagittarius A*. Does the center harbor a
supermassive black hole?

2) One approach is to determine whether stars in the central
region are moving very rapidly, as would be expected if a large
mass were present. During the past 5 years, two teams have
obtained high-resolution images of our Galactic center, each team
on several occasions, so that temporal changes in the positions
of stars could be detected. The observations were conducted at
infrared wavelengths, which penetrate the gas and dust between
Earth and the Galactic center (a distance of approximately 25,000
light years) much more readily than optical light. In summary,
the data are in excellent agreement with the conclusion that the
gravitational potential of the central region of our Galaxy is
dominated by a single object. The derived mass of this object is
(2.6 +- 0.2) x 10^(6) solar-masses, and the mass density within a
radius of 0.05 light-years is at least 6 x 10^(9) solar-masses
per cubic light-year, effectively eliminating all possibilities
other than a black hole.

3) Although our Galaxy provides the most convincing case for the
existence of supermassive black holes, observations of the
centers of a few other galaxies bolster the conclusion. For
example, very precise measurements of the galaxy NGC 4258 reveal
a central compact object with a derived mass 3.6 x 10^(7) solar-
masses. On somewhat larger scales, spectra obtained with the
Hubble Space Telescope show gas and stars rapidly moving in a
manner consistent with the presence of a supermassive black hole.
The most massive existing case, that of the giant elliptical
galaxy M87, is approximately 3 x 10^(9) solar-masses. Moreover,
x-ray observations of some active galactic nuclei reveal emission
from a hot disk of gas apparently very close to a black hole,
since extreme relativistic effects are detected. In general, it
now seems that a supermassive black hole is found in nearly every
large galaxy amenable to such observations.

4) The author concludes: "In the last decade of the 20th century,
black holes have moved firmly from the arena of science fiction
to that of science fact. Their existence in some *binary star
systems, and at the centers of massive galaxies, is nearly
irrefutable. They provide marvelous laboratories in which the
strong-field predictions of Einstein's general theory of
relativity can be tested."

Proc. Nat. Acad. Sci. http://www.pnas.org

--------------------------------

Notes by ScienceWeek:

Sagittarius A*: Sagittarius A is a prominent radio source in the
constellation Sagittarius, coincident with or close to the center
of our Galaxy. It is a highly complex region consisting of a
central core approximately 50 light-years in diameter.
Sagittarius A* is a compact component at the heart of the central
core of Sagittarius A. Sagittarius A* is an intense source of
radio waves, and is apparently unique in our Galaxy: while
everything else in our Galaxy is on the move as they follow their
orbits, Sagittarius A* is absolutely stationary and must
therefore lie exactly at the Galaxy's center. Many astronomers,
in fact, use Sagittarius A* as the "Greenwich Meridian" of the
Galaxy.

binary star systems: Binary stars are a pair of stars revolving
around a common center of mass under the influence of their
mutual gravitational attraction, and apparently the majority of
stars in the Universe are binaries and not singlets. In some
cases the binary system is resolvable into two components, and in
other cases the presence of a second star is inferred by
perturbations in the motion or emitted radiation of the first
star. If the binaries are close enough, they may share stellar
material, and this results in a particular kind of stellar
evolution. In the black hole-binary systems mentioned in this
report, matter transfers from a relatively normal star (known as
the "secondary star") to a dark compact object (the "primary").
Recent comparisons of x-ray and optical brightness suggest that
in many cases the dark primary in such a binary system is a black
hole.

ScienceWeek http://scienceweek.com