ScienceWeek

SCIENCEWEEK

ScienceWeek
May 16, 2003
Vol. 7 Number 20

An Online Digest of Research in the Sciences

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At the last dim horizon, we search among ghostly errors of
observations for landmarks that are scarcely more substantial.
The search will continue. The urge is older than history. It is
not satisfied and it will not be suppressed.
-- Edwin Hubble (1889-1953)

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Section 1

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Symposium: Astronomy: Galaxies

1. Introduction
2. The Milky Way
3. Galaxy Formation
4. Galaxy Evolution
5. Galaxy Clusters
6. Galaxy Encounters
7. Galaxies and Dark Matter
8. Active Galactic Nuclei

Notices and Subscription Information

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Section 2

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SYMPOSIUM: ASTRONOMY: GALAXIES

1. INTRODUCTION

SOME NOTES ON TERMINOLOGY

"Doppler-shift" is an observed change in spectrum frequencies
when the source of the spectrum or the observer move either
toward or away from each other. In astronomy, radial velocity is
the velocity of a star along the line of sight of an observer,
determined by measuring the Doppler-shift in the star's spectrum,
and periodic perturbations of an observed stellar Doppler-shift
have in some cases been interpreted as evidence for the existence
of a massive object orbiting the star.

"Redshift" (symbol: z) is a lengthening of the wavelengths of
electromagnetic radiation from a source caused either by the
movement of the source (Doppler effect) or by the expansion of
the universe (cosmological redshift). Redshift is defined as the
change in wavelength of a particular spectral line divided by the
unshifted wavelength of that line. Large redshifts imply large
radial velocities (which imply large distances, according to
current cosmological theory), but at redshifts greater than about
0.2 there is a relativistic divergence from a linear relation. A
redshift of 4.0 corresponds to an object receding with a radial
velocity 92% that of the velocity of light. The largest
astrophysical redshifts so far observed are of the order of z =
4.9. The furthest galaxy on record is at a redshift z ~ 5), which
implies a distance of approximately 15 billion light years.

A "Cepheid variable star" is a member of a group of yellow giant
or supergiant stars with pulsating luminosities, the periods of
the pulsations directly related to the absolute magnitudes of
these stars. The relation between period of pulsation and average
brightness of Cepheid variable stars was discovered during 1908-
1912 by Henrietta Swan Leavitt (1868-1928), a graduate of
Radcliffe College on the staff of the Harvard Observatory.
Leavitt discovered 2400 variable stars, doubling the number known
in her time. In the early years of stellar spectroscopy,
particularly at the Harvard Astronomical Observatory, nearly all
the data was catalogued and analyzed by female astronomers,
called "computers", who were forbidden because of their sex to
use the telescopes. It is an irony of the social history of
science that the work of such female astronomers as Henrietta
Swan Leavitt and Annie Jump Cannon (1863-1941) came to be of
greater significance than the work of many of the male
astronomers who considered these female astronomers to be no more
than menial assistants.

The term "planetary nebula" refers to any of a class of bright
nebulae that may resemble planets when viewed through a small
telescope but are in fact expanding shells of luminous gas far
outside the solar system. There are an estimated 20,000 objects
called "planetary nebulae" in our Galaxy, each planetary nebula
representing gas expelled relatively recently from a central star
very late in its evolution. (In contrast, diffuse and dark
nebulae are clouds of gas from which young stars form.) Planetary
nebulae are relatively small, having a typical radius of one
light-year and containing a mass of gas equivalent to
approximately 0.3 solar-mass. Such nebulae assume various shapes,
and one research problem is to explain their formation and
dynamics.

In 1929, Edwin Hubble announced a general law of redshifts now
known as the "Hubble law", the law stating that the velocity of
recession of a galaxy equals a constant times its distance. Thus,
the more distant a galaxy, the faster its apparent recession from
us. The constant, now denoted as H(sub0), is called the "Hubble
constant" (or Hubble parameter, since in some models it is time
dependent), is a critical quantity in all current cosmological
models, and enormous efforts have been made to determine its
exact value.

One of the central questions in cosmology is whether the universe
is a multi-dimensional equivalent of a 2-dimensional surface
("flat"), a sphere ("closed"), or a saddle ("open"). The
geometry, in the context of current theory and observations,
determines whether the universe will expand forever or eventually
collapse.

ON GALAXIES AND THE MILKY WAY

"Our modern picture of the universe dates back to only 1924, when
the American astronomer Edwin Hubble [1889-1953] demonstrated
that ours was not the only galaxy. There were in fact many
others, with vast tracts of empty space between them. In order to
prove this, he needed to determine the distances to these other
galaxies, which are so far away that, unlike nearby stars, they
really do appear fixed. Hubble was forced, therefore, to use
indirect methods to measure the distances. Now, the apparent
brightness of a star depends on two factors: how much light it
radiates (its luminosity), and how far it is from us. For nearby
stars, we can measure their apparent brightness and their
distance, and so we can work out their luminosity. Conversely, if
we knew the luminosity of stars in other galaxies, we could work
out their distance by measuring their apparent brightness. Hubble
noted that certain types of stars always have the same luminosity
when they are near enough for us to measure; therefore, he
argued, if we found such stars in another galaxy, we could assume
that they had the same luminosity -- and so calculate the
distance to that galaxy. If we could do this for a number of
stars in the same galaxy, and our calculations always gave the
same distance, we could be fairly confident of our estimate.

"In this way, Edwin Hubble worked out the distances to nine
different galaxies. We now know that our galaxy is only one of
some hundred thousand million that can be seen using modern
telescopes, each galaxy itself containing some hundred thousand
million stars... We live in a galaxy that is about one hundred
thousand light-years across and is slowly rotating; the stars in
its spiral arms orbit around its center about once every several
hundred million years. Our sun is just an ordinary, average-
sized, yellow star, near the inner edge of one of the spiral
arms. We have certainly come a long way since Aristotle and
Ptolemy, when we thought that the earth was the center of the
universe!"

Stephen W. Hawking: A Brief History of Time: From the Big Bang to
Black Holes. Bantam Press 1988, p.36

ON GALAXIES AND THE BIG BANG

"In the 1930s, a Belgian astrophysicist, Georges Lemaitre [1894-
1966], used the combination of observation and theory to come up
with the first version of what we would now call the Big Bang
model of the Universe. He talked in terms of a "primordial atom"
(or sometimes "primeval egg"), containing all the mass of all the
galaxies in the visible Universe, sitting alone in space and then
suddenly breaking apart in an explosion, like the fission of a
giant radioactive nucleus. This image encouraged other people to
take up the investigation of the Big Bang -- but in one respect
it is a misleading one, since what Einstein's equations tell us
is that space itself is expanding. The Big Bang was not an
explosion that took place somewhere in empty space, with
fragments from the explosion (galaxies) flying apart through
space like shrapnel from an exploding shell. Rather, what happens
is that space itself expands, and takes galaxies along for the
ride. It's like a piece of elastic on which you make several ink
blobs. When you pull the two ends apart, the elastic expands and
the blobs of ink move farther apart -- but they do not move
through the elastic.

"Hard though it may be to picture, what the general theory of
relativity tell us is that space and time were born, along with
matter, in the precursor to the Big Bang, and that this bubble of
spacetime full of matter and energy... has expanded ever since.
The galaxies fill the Universe today, and the matter they contain
always did fill the Universe, although obviously the pieces of
matter were closer together when the Universe was smaller. Since
the cosmological redshift is caused not by galaxies moving
through space but by space itself expanding in between the
galaxies, it is certainly not a Doppler effect, and it isn't
really measuring velocity, but a kind of pseudo-velocity. Partly
for historical reasons, partly for convenience, astronomers do,
though, continue to refer to the "recession velocities" of
distant galaxies, although no competent cosmologist ever
describes the cosmological redshift as a Doppler effect."

John Gribbin: Stardust: Supernovae and Life -- The Cosmic
Connection. Yale University Press 2000, p.115.

ON GALAXIES

"We can divide galaxies into three classes: elliptical, spiral,
and irregular -- with subclasses specifying the galaxy's shape.
The elliptical galaxies contain little gas and dust and few
bright, young stars. Spiral and irregular galaxies have large
amounts of gas and dust and are actively making new stars.

"To measure the properties of galaxies, we must first find out
how far away they are. For the nearer galaxies, we can judge
distances using distance indicators, objects whose luminosity or
diameter is known. The most accurate distance indicators are
Cepheid variable stars. Other distance indicators are bright
giants and supergiants, globular clusters, planetary nebulae, and
novae. Another type of distance indicator, the H II regions, have
known diameters. To use them, we compare their angular diameter
with their known linear diameter. In addition, we can estimate
the distance to the farthest galaxy clusters using the average
luminosity of the brightest galaxies.

"The Hubble law shows that the radial velocity of a galaxy is
proportional to its distance. Thus we can use the Hubble law to
estimate distances. The galaxy's radial velocity divided by the
Hubble constant equals its distance in megaparsecs.

"The masses of galaxies can be measured in two basic ways: the
rotation curve method and the velocity dispersion method. The
rotation curve method is more accurate but can be applied only to
nearby galaxies. Both methods suggest that galaxies contain 10 to
100 times more dark matter than visible matter.

"Galaxies occur in clusters. Our own galaxy is a member of the
Local Group, a small cluster. A galaxy in a rich cluster may
collide with other galaxies more often than a galaxy in a poor
cluster, and such collisions can force a galaxy to form new stars
and use up its gas and dust. Collisions can also strip gas out of
a galaxy. This may explain why elliptical and SO galaxies are
more common in rich clusters than in poor clusters. Spiral
galaxies may be star systems that have not experienced many
collisions.

"Clusters of galaxies are organized into super-clusters, and
superclusters are linked together in a network of filaments and
walls. The regions between seem to be great voids where there are
few if any galaxies. How these filaments, walls, and voids
developed is not yet understood."

Michael A. Seeds: Horizons: Exploring the Universe. Wadsworth
1995, p.300.

ON GALAXIES AND DARK MATTER

"We know that the curvature of space is logically intertwined
with the energy it contains. The general theory of relativity
tells us that the gravitational force, which has one energy act
on another, is nothing but a different name for the curvature of
space. Space altogether can be flat only if it contains just the
right amount of energy -- or if there is a conspiratory
cancellation of different effects... But this condition is not
met as long as we consider only the energy of all matter that, to
our knowledge, is present in the universe: The visible matter --
that is, that matter that is observable through its light
emission or reflection -- contains only about 1 percent of the
"critical" mass that is needed for an overall flat space. "Light"
in this connotation stands for electromagnetic radiation of any
wavelength that reaches Earth -- from optical to gamma rays.

"It is not only the theory of inflation that tells us we don't
know of all of the masses in the universe. In addition, there are
observed facts that tell us the same. One of these is the chaotic
motion of galaxies inside galaxy clusters. Galaxies don't appear
singly, but rather inside such larger groupings. They are kept
from escaping by the gravitational pull of the mass of the
cluster. Individual galaxies move inside a cluster with
velocities that are measurable by means of the Doppler effect.
That's how we know these velocities to be so large that the
gravity of the visible mass of the cluster does not suffice to
hold the galaxies it contains together: Were there only the
cluster's visible mass, its galaxies would have had to fly apart
long ago. To keep all of them within the cluster, there must be
about one hundred times more mass present than what is noticeable
to us as visible matter.

"Even the most remote stars close to the outer rim of a galaxy
like ours, the Milky Way, orbit the center of the galaxy at a
velocity that would make them take off into intergalactic space
if that galaxy contained only the matter that is visible to the
astronomer. Just to hold the stars within a galaxy, we need the
gravitational pull of at least ten times more mass than is
visible.

"There are arguments in favor of the assumption that the
invisible matter that holds the galaxies together consists of the
same atoms as visible matter. We call it baryonic matter. Baryons
(after the Greek word for heavy) are the massive constituents of
nuclear matter, mostly the protons and neutrons... The planet
Jupiter could serve as an example for a fairly large
concentration of dark baryonic matter that becomes noticeable
mainly through its gravitational pull. Burnt-out stars and
nonluminous intergalactic clouds of dust or gas may also
contribute to dark matter. There are a number of good reasons for
astrophysicists to assume that all luminous galaxies are embedded
in large spherical regions filled with a ten-times-larger mass of
such composition.

"The process has to be repeated on the next larger scale: To keep
galactic clusters from flying apart, it takes more dark matter.
The larger the regions we take under consideration, the more dark
matter is needed for a flat universe. All in all, we come to the
conclusion that no more than 1 percent of all matter in the
universe falls in the visible category."

Henning Genz: Nothingness: The Science of Empty Space. Perseus
Publishing 1998, p.298.

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2. THE MILKY WAY

A STAR IN A 15.2-YEAR ORBIT AROUND THE SUPERMASSIVE BLACK HOLE AT
THE CENTER OF THE MILKY WAY

R. Sch”del et al (Max-Planck-Institut f�r extraterrestrische
Physik Garching, DE) discuss the center of the Milky Way, the
authors making the following points:

1) Many galaxies are thought to have supermassive black holes at
their centers1 -- more than a million times the mass of the Sun.
Measurements of stellar velocities(2-5) and the discovery of
variable X-ray emission have provided strong evidence in favor of
such a black hole at the centre of the Milky Way, but have
hitherto been unable to rule out conclusively the presence of
alternative concentrations of mass.

2) For the past ten years, the authors have been carrying out
high-resolution near-infrared imaging and spectroscopy of the
central few light years of our Milky Way for a detailed study of
the stellar dynamics in the vicinity of the compact radio source
SgrA* (2,3,5), the most likely counterpart of the putative black
hole. From a statistical analysis of the stellar proper motions
(velocities on the plane of the sky derived from multi-epoch
imaging data) and line-of-sight velocities (Doppler motions
derived from spectral lines) the authors deduced the presence of
a mass of about 2.6 to 3.3 million solar masses (M) concentrated
within ten light days of SgrA* (2,3,5). To further improve the
sensitivity (by about 20) and the angular resolution/astrometric
precision of the study (by about 3), the authors began this year
to use the new COud‚ near-infrared camera (CONICA)/Nasmyth
adaptive optics system (NAOS) imager/spectrometer on the 8-m UT4
(Yepun) of the European Southern Observatory (ESO) Very Large
Telescope (VLT)11-13.

3) In summary: The authors report ten years of high-resolution
astrometric imaging that allows them to trace two-thirds of the
orbit of the star currently closest to the compact radio source
(and massive black-hole candidate) Sagittarius A*. The
observations, which include both pericentre and epicenter
passages, show that the star is on a bound, highly elliptical
Keplerian orbit around Sgr A*, with an orbital period of 15.2
years and a epicenter distance of only 17 light hours. The orbit
with the best fit to the observations requires a central point
mass of (3.7 +- 1.5) x 10^(6) solar masses (M). The authors
suggest the data no longer allow for a central mass composed of a
dense cluster of dark stellar objects or a ball of massive,
degenerate fermions.

References (abridged):

1. Kormendy, J. & Richstone, D. Inward bound--The search for
supermassive black holes in galactic nuclei. Annu. Rev. Astron.
Astrophys. 33, 581-624 (1995)

2. Eckart, A. & Genzel, R. Observations of stellar proper motions
near the Galactic Center. Nature 383, 415-417 (1996)

3. Genzel, T., Eckart, A., Ott, T. & Eisenhauer, F. On the nature
of the dark mass in the center of the Milky Way. Mon. Not. R.
Soc. 291, 219-234 (1997)

4. Ghez, A., Klein, B. L., Morris, M. & Becklin, E. E. High
proper-motion stars in the vicinity of Sagittarius A*: Evidence
for a supermassive black hole at the center of our galaxy.
Astrophys. J. 509, 678-686 (1998)

5. Genzel, R., Pichon, C., Eckart, A., Gerhard, O. & Ott, T.
Stellar dynamics in the Galactic Center: proper motions and
anisotropy. Mon. Not. R. Soc. 317, 348-374 (2000)

Nature 2002 419:694

Related Background:

RAPID X-RAY FLARING FROM THE DIRECTION OF THE SUPERMASSIVE BLACK
HOLE AT THE GALACTIC CENTER

F.K. Baganoff et al (Massachusetts Institute of Technology, US)
discuss the Galactic Center, the authors making the following
points:

1) The nuclei of most galaxies are now believed to harbor
supermassive black holes(1). The motions of stars in the central
few light years of our Milky Way Galaxy indicate the presence of
a dark object with a mass of about 2.6 x 10^(6) solar masses
(2,3). This object is spatially coincident with the compact radio
source Sagittarius A* (Sgr A*) at the dynamical center of the
Galaxy, and the radio emission is thought to be powered by the
gravitational potential energy released by matter as it accretes
onto a supermassive black hole(4,5). Sgr A* is, however, much
fainter than expected at all wavelengths, especially in X-rays,
which has cast some doubt on this model. The first strong
evidence for X-ray emission was found only recently.

2) Our view of Sgr A* in the optical and ultraviolet wavebands is
blocked by the large visual extinction caused by dust and gas
along the line of sight. Sgr A* has not been detected in the
infrared owing to its faintness and to the bright infrared
background from stars and clouds of dust. We thus need to detect
X-rays from Sgr A* in order to constrain the spectrum at energies
above the radio-to-submillimeter band and to test whether gas is
accreting onto a supermassive black hole.

3) In summary: The authors report the discovery of rapid X-ray
flaring from the direction of Sgr A*, which, together with the
previously reported steady X-ray emission, provides compelling
evidence that the emission is coming from the accretion of gas
onto a supermassive black hole at the Galactic Center.

References (abridged):

1. Richstone, D. et al. Supermassive black holes and the
evolution of galaxies. Nature 395 (suppl. on optical astronomy)
A14-A19 (1998)

2. Genzel, R., Pichon, C., Eckart, A., Gerhard, O. E. & Ott, T.
Stellar dynamics in the Galactic Center: proper motions and
anisotropy. Mon. Not. R. Astron. Soc. 317, 348-374 (2000)

3. Ghez, A. M., Morris, M., Becklin, E. E., Tanner, A. &
Kremenek, T. The accelerations of stars orbiting the Milky Way's
central black hole. Nature 407, 349-351 (2000)

4. Lynden-Bell, D. & Rees, M. J. On quasars, dust and the
Galactic Centre. Mon. Not. R. Astron. Soc. 152, 461-475 (1971)

5. Melia, F. & Falcke, H. The supermassive black hole at the
Galactic Center. Annu. Rev. Astron. Astrophys. 39, 309-352 (2001)

Nature 2001 413:45

Related Background:

GENESIS OF THE HEAVIEST ELEMENTS IN THE MILKY WAY GALAXY

C. Sneden and J. Cowan (University of Texas Austin, US) discuss
the Milky Way galaxy, the authors making the following points:

1) Most beginning chemistry students struggling with the
complexities and underlying structure of the periodic table of
the elements will simply accept the existence of the
approximately 90 stable elements. Rarely does it occur to them
that, somewhere and in some way, all of the elements had to be
synthesized. Such element generation or nucleosynthesis, through
transmutation of one element into another, is a crucial by-
product of stellar energy generation. It has occurred since the
birth of the first stars in the Galaxy, and without it life on
Earth would not be possible. The authors review the observations
and interpretations of heavy elements in the oldest stars, born
10 to 15 billion years ago (Ga). These old and metal-deficient
stars exhibit surprisingly large variations in concentrations of
heavy elements. Understanding these variations promises to
improve our understanding of the origin of the elements.

2) The chemical composition of primordial solar system material
is derived from analyses of the solar spectrum and carbonaceous
chondrite meteorites [(1) and references therein]. This elemental
abundance pattern is used as a reference standard to compare with
data from other stars, and we find it repeated so often among
stars of the galactic disk that it is often considered as the
cosmic or universal chemical composition. In this abundance
distribution, H and He account for nearly all of the ordinary
matter; the rest of the periodic table provides only trace
elements. Among the remaining elements, the distribution features
rapidly declining abundances with increasing atomic number Z, an
odd/even pattern with even-Z elements being more abundant than
their immediate odd-Z neighbors, and relative abundance peaks
that correspond to the more tightly bound atomic nuclei.
H and He were created wholly or substantially in nuclear
reactions that accompanied the Big Bang. Li also was made in the
Big Bang and can sometimes be synthesized late in stellar
lifetimes. The trio of Li, Be, and B are easily destroyed via
proton capture in the interiors of ordinary stars, but can be
made in high-energy cosmic ray or neutrino spallation reactions
on C, N, and O target nuclei in the interstellar medium (ISM).
All other elements owe their existence to nuclear reactions in
stellar interiors.

3) Charged-particle fusion reactions account for the abundances
of nearly all of the isotopes of elements through the Fe group.
These reactions are generally exothermic and thus ultimately are
the energy sources that power the luminosities of the stars. The
most common reactions are those that fuse four H nuclei into a He
nucleus and fuse three He nuclei into a C nucleus. Most stars
such as the Sun are of relatively low mass and hence have
interior temperatures and densities sufficient to create only the
lighter elements. In the last stages and death throes of the
rare, more massive stars (8M, where M is the mass of the Sun),
the interior temperatures and pressures are high enough to form
Fe-group nuclei, but the fusion process is essentially the same
as that which turns H into He.

4) In summary: The authors review the origin and evolution of the
heavy elements, those with atomic numbers greater than 30, in the
early history of the Milky Way. There is a large star-to-star
bulk scatter in the concentrations of heavy elements with respect
to the lighter metals, which suggests an early chemically unmixed
and inhomogeneous Galaxy. The relative abundance patterns among
the heavy elements are often very different from the Solar System
mix, revealing the characteristics of the first element donors in
the Galaxy. Abundance comparisons among several halo stars show
that the heaviest neutron-capture elements (including barium and
heavier) are consistent with a scaled Solar System rapid neutron-
capture abundance distribution, whereas the lighter such elements
do not conform to the solar pattern. The stellar abundances
indicate an increasing contribution from the slow neutron-capture
process (s-process) at higher metallicities in the Galaxy. The
detection of thorium in halo and globular cluster stars offers a
promising, independent age-dating technique that can put lower
limits on the age of the Galaxy.(2-5)

References (abridged):

1. N. Grevesse and A. J. Sauval, Space Sci. Rev. 85, 161 (1998)

2. The high temperatures that would be required (to overcome the
electric Coulomb barriers) for fusion of nuclei beyond iron would
also result in a large number of high-energy photons. These
photons in turn result in photodisintegration of nuclei that
suppress any possible charge-particle fusion reactions.

3. H fusion in stars occurs in the inner core (approximately the
innermost 10% of the star by mass) and lasts for about 90% of the
star's total life. After the exhaustion of H in the core, the
star will then develop a thin (thousands of km) H fusion shell
outside of the inert He core. Later, when the temperature rises
to above 100 million K, the He core will fuse C and O. When the
He is finally depleted in the core, a thin He fusion shell,
outside of the now C/O core but interior to the H fusion shell,
will ignite. These later fusion stages occur only during the last
~10% of a star's life.

4. Supernovae (SNe) are observationally categorized by the
presence (type II) or absence (type I) of hydrogen spectral
lines. Further, type II SNe are normally thought to result from
the collapse and explosion of single massive, short-lived stars.
Type I SNe are thought to be phenomena of lower-mass, longer-
lived binary star systems, with the eventual explosion and
complete destruction of the white dwarf member of the binary.

5. A. G. W. Cameron, in Essays in Nuclear Astrophysics, C. A.
Barnes, D. D. Clayton, D. N. Schramm, Eds. (Cambridge Univ.
Press, Cambridge, 1982), pp. 23-43.

Science 2003 299:70

Related Background:

ON THE BLACK HOLE AT THE CENTER OF OUR GALAXY

Recent observations have led to the conclusion that at the center
of many galaxies there is an object producing effects
characteristic of a supermassive *black hole. Alexei V.
Filippenko (University of California Berkeley, US) reviews
current research on black holes, the author making the following
points concerning the apparent massive black hole at the center
of our own Galaxy:

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? 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.

2) 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.

3) 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. 1999 96:9993

Notes:

*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://www.scienceweek.com

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3. GALAXY FORMATION

ON THE MYSTERY OF GALAXY FORMATION

Marco Scodeggio (Istituto di Fisica Cosmica Milano, IT) discusses
galaxy formation, the author making the following points:

1) There are two basic models for galaxy formation. In the
monolithic collapse scenario, all galaxies were formed in a
single event, through the gravitational collapse of a cloud of
primordial gas, very early in the history of the universe (1,2).
In the hierarchical merging scenario, galaxies are gradually
assembled through multiple mergers of smaller subgalactic units,
a process that continues from the early universe to the current
epoch (3,4).

2) These differences extend to ideas about galaxy evolution. In
the monolithic collapse scenario, galaxies of different
morphological types (spirals and ellipticals) are born
intrinsically different, whereas in the hierarchical merging
scenario, galaxies end up as spirals or ellipticals depending on
the details of their merger history. As a result, the first model
predicts that the number of galaxies of a given type should be
approximately constant at all redshifts (that is, throughout the
history of the universe), whereas the second predicts that there
number should decrease with increasing redshift (that is,
decreasing age).

3) Attempts to discriminate between the two models focus mostly
on elliptical galaxies, which are easier to study than spiral
ones. Present-epoch ellipticals form a very homogeneous family
with very similar intrinsic properties. Compared with the
heterogeneous family of spiral galaxies, ellipticals in the local
universe have little or no dust, gas, and star formation activity
(5). Furthermore, they are mostly if not exclusively composed of
an old stellar population, about as old as the universe, with
very similar relative ages. This fact is responsible for the most
distinctive property of ellipticals: their color. Ellipticals are
the reddest galaxies in the local universe (5).

4) Neither galaxy formation model can be discarded convincingly,
although, until recently, the monolithic collapse scenario had to
contend with one important, albeit indirect, piece of evidence
against it. If elliptical galaxies all formed at high redshift in
a single event, for a short period they must have had very strong
star formation activity. Simple model calculations indicate that
galaxies with so many young and bright stars should be luminous
enough to be observable with current telescopes, despite their
large distances. But they were never observed.

References (abridged):

1. O. J. Eggen, D. Lynden-Bell, A. Sandage, Astrophys. J. 136,
748 (1962)

2. R. B. Larson, Mon. Not. R. Astron. Soc. 166, 585 (1974)

3. S. D. M. White, M. J. Rees, Mon. Not. R. Astron. Soc. 183, 341
(1978)

4. S. Cole et al., Mon. Not. R. Astron. Soc. 271, 781 (1994)

5. M. Roberts, M. P. Haynes, Annu. Rev. Astron. Astrophys. 32,
115 (1994)

Science 2001 294:537

Related Background:

ON LOCAL AND DISTANT GALAXIES

Francesco Bertola (University of Padova, IT) discusses galaxies,
the author making the following points:

1) In our expanding universe, radiation emitted by astronomical
objects appears more redshifted the farther the objects are from
us. Galaxies close to our own galaxy have low redshift and are
relatively old; galaxies at high redshift are distant and hence
young. The advent of the Hubble Space Telescope and of large (8
to 10 meters) ground-based telescopes during the last decade has
greatly facilitated the study of distant young galaxies.
Comparison of the local universe with the early universe is
providing insights into how galaxies have evolved on a
cosmological time scale.

2) The latest value for the mass of our galaxy's dark halo (which
holds most of the galaxy's mass) is about 2 x 10^(12) solar
masses. Just 15 years ago, the best estimate for the total mass
of our galaxy was an order of magnitude lower. The current value
has been derived from state-of-the-art data for the radial
velocities of the globular clusters (gravitationally bound
concentrations of 10,000 to 1 million stars) that surround our
galaxy and of nearby "satellite" galaxies. These objects serve as
tracers for our galaxy's gravitational potential and hence its
mass. A much higher accuracy will be achieved when the radial and
transverse velocities of the satellite galaxies have been
determined by GAIA, a space mission to be launched in 2010.

3) A lively current debate in the astronomy community concerns
the processes leading to the formation of galaxies. The key
question is whether all galaxies formed early on through
gravitational collapse in a "monolithic collapse" event and have
since evolved in isolation, or whether they are the result of
successive mergers between ever larger structures ("hierarchical
merging"). These models lead to different distributions of dark
mass in galaxies. Numerical simulations suggest that in the
hierarchical merging scenario, dark matter should peak in the
centers of galaxies. Systematic study of the rotation curve of
low-surface-brightness galaxies, which are believed to be
dominated by dark matter, reveals that the density distribution
is better fitted by a model with a central constant density core
than with a peaked distribution, suggesting that more efforts are
needed to reconcile simulation and observation.

References (abridged):

1. The Mass of Galaxies at Low and High Redshift, workshop
organized by ESO and the Universit„ts-Sternwarte M�nchen, Venice,
24 to 26 October 2001; see http://www.eso.org/gen-
fax/meetings/gmass2001

2. M. I. Wilkinson, N. W. Evans, Mon. Not. R. Astron. Soc. 310,
645 (1999). GAIA stands for Global Astrometric Interferometer for
Astrophysics

3. R. P. Olling, M. Merrifield, Mon. Not. R. Astron. Soc. 311,
361 (2000)

4. R. P. Olling, Mon. Not. R. Astron. Soc. 326, 164 (2001)

5. S. S. McGaugh, V. C. Rubin, W. J. G. de Blok, Astron. J. 122,
2381 (2001)

Science 2002 295:283

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4. GALAXY EVOLUTION

ON THE EVOLUTION OF GALAXIES

In a detailed review of research in this century concerning the
evolution of galaxies, Alan Dressler (Carnegie Observatories
Pasadena, US) makes the following points:

1) When the century opened, most astronomers assumed the Universe
was eternal and basically changeless, its general structure
immutable. In 1915, with the publication of Einstein's general
theory of relativity, there were implications of the cosmic role
of gravity, but these implications were for the most part
ignored. Indeed, even after 1924 and the proof by *Edwin Hubble
(1889-1953) that the spiral nebulae are other galaxies at vast
distances, astronomers were slow to recognize the implications of
the new observations.

2) The first big step in changing the view of the cosmos was the
construction by *George Ellery Hale (1868-1938) of the 100-inch
Hooker reflector on Mount Wilson (US), a project completed in
1918. This was the telescope used by Hubble and his colleagues to
reveal the large-scale organization of the Universe into
galaxies, the vast size of the Universe, and the expansion of the
Universe. *George Lemaitre (1894-1966) soon proposed that the
expansion of the Universe implied a dense explosive birth of the
Universe at a specific finite time in the past (the event that
came to be called the Big Bang).

3) An even greater telescope was needed, and again George Hale
led the way in the building of the 200-inch reflector on Palomar
Mountain (US), that instrument finally completed in 1948. The
200-inch telescope produced the first observations of galaxy
evolution -- the first evidence that galaxies observed at high
redshift are unlike galaxies closer to us in time. An even
greater accomplishment of the 200-inch telescope was a series of
observations concerning the spectra of *quasars, and the evidence
for their immense distances and luminosities. Theorists
eventually proposed that quasars were *black holes of 100 million
solar-masses or more. It is now clear that most galaxies with a
central bulge, including our own Galaxy, harbor massive black
holes at their cores.

4) In 1961, Allan Sandage published a landmark paper outlining
the possibility of testing cosmological models with the 200-inch
telescope, and over the next two decades Sandage devoted himself
to this project. Unfortunately, the underlying premise of the
project -- that the brightest galaxy in every galactic cluster
has about the same true luminosity -- was demonstrated by
*Beatrice Tinsley (1941-1981) to be untenable.

5) In the fall of 1977, at a Yale University (US) conference on
the evolution of galaxies, Harvey Butcher and Augustus Oemler
presented their evidence for relatively young star-forming
galaxies. This evidence, which implied strong galaxy evolution
during relatively recent cosmic time, met with controversy and
skepticism.

6) In the 1980s, observations by various groups proved that
Butcher and Oemler were correct, and it was now understood that
these relatively young galaxies were often producing new stars in
huge bursts. These bursting galaxies are evidently spirals with a
more disheveled appearance than is common today, and in their
twisted and distorted disks huge numbers of stars were recently
born.

7) During the past 2 years, among the most interesting results of
various observations with various instruments is the formulation
of the so-called Madau diagram (popularized by Piero Madau) that
plots the Universe-wide rate of star formation from early times
to today, spanning almost the whole history of the cosmos. The
rate of star formation apparently rose rapidly in the first few
billion years, the peak rate at about 5 or 6 billion years later
at redshifts of 1 to 2. (Our Sun apparently formed at a time
corresponding approximately to redshift = 0.5.) The author
concludes: "Our generations are fortunate to live to see one of
the great mysteries of where we came from in process of being
solved."

Sky & Telescope 1998 October

Notes:

*Edwin Hubble (1889-1953): Hubble first studied law before
switching to astronomy at the age of 25. He began his work at the
Mount Wilson Observatory with the 100-inch telescope at the age
of 30. In 1941, at the age of 52, he tried to join the US Army to
fight the Nazis, but he was persuaded that he could do more in
war-related research.

*George Ellery Hale (1868-1938): Hale is best known for his work
building large-telescopes (and for obtaining the funds for the
Yerkes Observatory, named after the street-car magnate Charles
Tyson Yerkes), but already at the age of 21 he invented the
spectroheliograph, a device that made it possible to photograph
the light of a single spectral line of the sun, and he made
several ground-breaking observations with this instrument.

*George Lemaitre (1894-1966): Lemaitre began his professional
life as a civil engineer, then at 21 he switched to physics and
mathematics. He also became a Roman Catholic priest at the age of
22. After obtaining his PhD at the Massachusetts Institute of
Technology in 1927, he settled in Belgium as a professor of
astrophysics at the University of Louvain. At the time of his
death, he was president of the Pontifical Academy of Sciences at
Rome. Lemaitre's theoretical ideas concerning the origin of the
Universe were published in 1927, when he was 31, but the paper
was largely unnoticed until the astrophysicist Arthur Eddington
(1882-1944) called attention to it much later.

*quasars: (quasi-stellar objects). Extremely luminous sources
radiating energy over the entire spectrum from x-rays to radio
waves, and which are apparently the oldest and most distant
objects in the universe.

*black holes: If the terminal stages of star death leave a
remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable. Another view of a black hole
is that it is a mass that has collapsed to such a small volume
that its gravity prevents the escape of all radiation. Space and
time essentially have no meaning in a black hole. The boundary of
the black hole is called the "event horizon", because any event
within the boundary is invisible outside, the invisibility
resulting from the fact that no radiation can escape to be
detected. The radius of the black hole depends upon how much
matter has fallen into the region; it is called the "Schwarzchild
radius", and it is usually a few kilometers. However, massive
black holes are possible and are thought to be the source of
quasars. If quasars indeed involve black holes, the radiation is
from material just outside the black hole, and not from anything
within it. Nothing inside a black hole can get out of it.

*Beatrice Tinsley (1941-1981): During her short life, Tinsley
managed to be a force in astronomy from her first entry into the
field. At the age of 25, an unknown graduate student at the
University of Texas, she rose before an audience about to hear
Allan Sandage and publicly challenged his idea that giant
elliptical galaxies exhibited luminosities constant enough to be
used as "standard candles" to estimate distances. She proved her
point by the age of 36, and the variability of galaxy
luminosities became the consensus view. It was Tinsley who co-
hosted the 1977 Yale conference that set the course of galaxy-
evolution studies. She died 4 years later of cancer. Near the
end, she wrote the following: "Let me be like Bach, creating
fugues; till suddenly the pen will move no more."

Related Background:

THE MORPHOLOGICAL EVOLUTION OF GALAXIES

R.G. Abraham and S. van den Bergh (University of Toronto, CA)
discuss galaxy evolution, the authors making the following
points:

1) Nearby galaxies are usually classified on the basis of a
scheme originally proposed by Edwin Hubble in 1926 (1). This
"tuning fork" classification system characterizes galaxies with
reference to a set of bright nearby standard galaxies. In
Hubble's scheme, galaxies are divided into ellipticals and
spirals. Spiral galaxies are subdivided into unbarred (S) and
barred (SB) categories, which define the tines of the tuning
fork. Along each tine, galaxies are further subdivided according
to the tightness and fine structure of their spiral arms, which
changes monotonically along the tuning fork in step with the
fraction of light in the central bulge of the galaxy. These
subcategories are denoted Sa, Sb, and Sc (SBa, SBb, SBc in the
case of barred spirals). A catch-all category for irregular
galaxies is also included.

2) More than 90% of luminous nearby galaxies fit within this
system. Intrinsically faint galaxies, such as many of the dwarf
galaxies in the Local Group (the agglomeration of galaxies within
1.3 Mpc of the Milky Way Galaxy), cannot be slotted into the
standard classification system. Such faint dwarf galaxies vastly
outnumber the bright galaxies described by the Hubble sequence.
However, these dwarfs contribute little to the total mass budget
of the galaxy population and they are difficult to detect at
large distances.

3) Modern attempts to understand the physical meaning of Hubble's
classification system are based on the idea that most matter in
the Universe is not in stellar or gaseous form, but is instead
composed of dark matter. Dark matter does not emit or absorb
radiation, and it can only be detected by its gravitational
effects on spectroscopically observed galaxy rotation curves or
by gravitational bending of galaxy images ("gravitational
lenses"). Some fraction of the dark matter in the Universe is
made up of baryons (protons, neutrons, and related particles),
but from the relative abundances of elements formed in the Big
Bang, well over 90% must be in a nonbaryonic form (2).
Theoretical work relies on the hypothesis that dark matter and
galaxies are linked, because gravitating concentrations of dark
matter originating soon after the Big Bang are responsible for
the formation of galaxies. In this view, small concentrations of
dark matter grow slowly at first, and are then gradually
compressed by self-gravity. However, when the concentrations
reach a critical density (about 200 times the mean background
density of the Universe), they undergo a catastrophic nonlinear
collapse, which results in the formation of an extended "halo" of
dark matter. Over time these halos clump together under their
mutual gravitational attraction, merging to form a hierarchy of
larger halos. The rate of cooling and angular momentum of
hydrogen gas drawn into these large halos governs the assembly of
normal galaxies and, ultimately, their morphology.

4) In summary: Many galaxies have taken on their familiar
appearance relatively recently. In the distant Universe, galaxy
morphology deviates significantly (and systematically) from that
of nearby galaxies at redshifts (z) as low as 0.3. This
corresponds to a time ~3.5 ž 10^(9) years in the past, which is
only ~25% of the present age of the Universe. Beyond z = 0.5 (5 ž
10^(9) years in the past), spiral arms are less well developed
and more chaotic, and barred spiral galaxies may become rarer. At
z = 1, around 30% of the galaxy population is sufficiently
peculiar that classification on Hubble's traditional "tuning
fork" system is meaningless. On the other hand, some
characteristics of galaxies have not changed much over time. The
space density of luminous disk galaxies has not changed
significantly since z = 1, indicating that although the general
appearance of these galaxies has continuously changed over time,
their overall numbers have been conserved.(3-5)

References (abridged):

1. E. P. Hubble, Astrophys. J. 64, 321 (1926)

2. In the last 3 years, astronomers have also found evidence for
"dark energy," a counterpart to dark matter that may be
responsible for an accelerating expansion of the Universe.

3. J. Peacock, Cosmological Physics (Cambridge Univ. Press,
Cambridge, 1999)

4. A. Balbi et al., Astrophys. J. 545, L1 (2000)

5. P. de Bernardis et al., Nature 404, 955 (2000)

Science 2001 293:1273

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5. GALAXY CLUSTERS

A RECONSTRUCTION OF THE INITIAL CONDITIONS OF THE UNIVERSE BY
OPTIMAL MASS TRANSPORTATION

U. Frisch et al (CNRS, FR) discuss the early Universe, the
authors making the following points:

1) Reconstructing the density fluctuations in the early Universe
that evolved into the distribution of galaxies we see today is a
challenge to modern cosmology(1). An accurate reconstruction
would allow us to test cosmological models by simulating the
evolution starting from the reconstructed primordial state and
comparing it to observations. Several reconstruction techniques
have been proposed(2-5), but they all suffer from lack of
uniqueness because the velocities needed to produce a unique
reconstruction usually are not known.

2) The authors demonstrate that reconstruction can be reduced to
a well-determined problem of optimization, and present a specific
algorithm that provides excellent agreement when tested against
data from N-body simulations. By applying this algorithm to the
redshift surveys now under way, one will be able to recover
reliably the properties of the primeval fluctuation field of the
local Universe, and to determine accurately the peculiar
velocities (deviations from the Hubble expansion) and the true
positions of many more galaxies than is feasible by any other
method.

References (abridged):

1. Narayanan, V. K. & Croft, R. A. Recovering the primordial
density fluctuations: a comparison of methods. Astrophys. J. 515,
471-486 (1999)

2. Peebles, P. J. E. Tracing galaxy orbits back in time.
Astrophys. J. 344, L53-L56 (1989)

3. Weinberg, D. H. Reconstructing primordial density
fluctuations--I. Method. Mon. Not. R. Astron. Soc. 254, 315-342
(1992)

4. Nusser, A. & Dekel, A. Tracing large-scale fluctuations back
in time. Astrophys. J. 391, 443-452 (1992)

5. Croft, R. A. & Gazta¤aga, E. Reconstruction of cosmological
density and velocity fields in the Lagrangian Zel'dovich
approximation. Mon. Not. R. Astron. Soc. 285, 793-805 (1997)

Nature 2002 419:675

Related Background:

X-RAY CLUSTERS OF GALAXIES AS TRACERS OF STRUCTURE IN THE
UNIVERSE

S. Borgani and L. Guzzo (University of Trieste, IT) discuss
galaxy clusters, the authors making the following points:

1) The present-day appearance of the galaxy distribution is the
result of the gravitational growth of the initial fluctuations
laid down during the very early stages of cosmic history, coupled
to dissipative processes which lit up matter and made it visible
to our telescopes. Cosmological models directly predict the
distribution and gravitational growth of the mass, not of the
light we actually observe from galaxies. Thus, the challenge of
unveiling the nature of the cosmic initial conditions from the
observed structure of the Universe is a process which is ideally
characterized by two steps. The first involves constructing maps
of the distribution of luminous objects on sufficiently large
scales to encompass a representative portion of the whole
Universe. The second requires relating such "light maps" to the
underlying mass through a physically motivated and robust recipe,
the two distributions being, in principle, not related by a one-
to-one correspondence. Although galaxy surveys are now able to
probe the distribution of luminous structures over scales larger
than 100 megaparsecs (Mpc, 1 pc = 3.26 light years), their
relation to the actual mass distribution depends on the still
poorly understood physics of galaxy formation and evolution.

2) Clusters of galaxies, the most evident concentrations in
galaxy maps, can themselves be used as tracers of the large-scale
structure of the Universe. Although they miss the fine details,
they can be used efficiently to study extremely large volumes at
a reduced cost in terms of telescope time with respect to fully
sampled galaxy surveys. Already the earliest statistical samples
of clusters from visually compiled catalogues(1-3) reached
typical depths of a few hundreds of Mpc, and the Abell catalogue
in particular1 still represents one of the main resources for
cosmological studies(4,5). It is within the "gravitational sinks"
of galaxy clusters that evidence for the elusive dark matter was
first found in the 1930s, as a necessary ingredient to explain
the fast motions observed for cluster galaxies.

3) The loose definition of a cluster as a collection of galaxies
is however intrinsically uncertain, and is certainly not optimal
for estimating its mass, as is required for linking observations
and theoretical predictions. Fortunately, about 20 30% of the
optically invisible mass of a cluster is in the form of a diffuse
hot gas8, trapped and heated to a temperature of the order of
10^(8) K by its gravitational potential. At such high
temperatures, this gas is a fully ionized plasma, producing
powerful X-ray emission by free free electron ion interactions,
the so-called "bremsstrahlung radiation".

4) In summary: Clusters of galaxies are visible tracers of the
network of matter in the Universe, marking the high-density
regions where filaments of dark matter join together. When
observed at X-ray wavelengths these clusters shine like cosmic
lighthouses, as a consequence of the hot gas trapped within their
gravitational potential wells. The X-ray emission is linked
directly to the total mass of a cluster, and so can be used to
investigate the mass distribution for a sizeable fraction of the
Universe. The picture that has emerged from recent studies is
remarkably consistent with the predictions for a low-density
Universe dominated by cold dark matter.

References (abridged):

1. Abell, G. O. The distribution of rich clusters of galaxies.
Astrophys. J. Suppl. 3, 211-278 (1958)

2. Abell, G. O., Corwin, H. G.Jr & Olowin, R. P. A catalog of
rich clusters of galaxies. Astrophys. J. Suppl. 70, 1-138 (1989)

3. Zwicky, F., Herzog, E., Wild, P., Karpowicz, M. & Kowal, C.
Catalogue of Galaxies and of Clusters of Galaxies (California
Institute of Technology, Pasadena, 1961-68)

4. Retzlaff, J., Borgani, S., Gottl”ber, S., Klypin, A. & M�ller,
V. Constraining cosmological models with cluster power spectra.
New Astron. 3, 631-646 (1998)

5. Miller, C. J. & Batuski, D. J. The power spectrum of rich
clusters to scales approaching 1000 Mpc. Astrophys. J.
(submitted); also preprint astro-ph/0002295 at xxx.lanl.gov
(2000).

Nature 2001 409:39

Related Background:

A MEASUREMENT OF THE COSMOLOGICAL MASS DENSITY FROM CLUSTERING IN
THE 2DF GALAXY REDSHIFT SURVEY

J.A. Peacock et al (University of Edinburgh, UK) discuss galaxy
clusters, the authors making the following points:

1) Hubble showed in 1934 that the pattern of galaxies on the sky
is non-random(1), and successive years have seen ever more
ambitious attempts to map the distribution of visible matter on
cosmological scales. In order to obtain a three-dimensional
picture, redshift surveys use Hubble's law to infer approximate
radial distances to a set of galaxies. The first major surveys of
this sort took place in the early 1980s (2-5), and were limited
to a few thousand redshifts, owing to the limited speed of
single-object spectroscopy. In the 1990s, redshift surveys were
extended to much larger volumes by a "sparse sampling" strategy.
These studies established that the Universe was close to uniform
on large scales but with a complex nonlinear supercluster network
of walls, filaments and voids on smaller scales.

2) The origin of this large-scale structure is one of the key
issues in cosmology. A plausible assumption is that structure
grows by gravitational collapse of density fluctuations that are
small at early times -- but it is essential to test this idea.
One important signature of gravitational instability is that
collapsing structures should generate "peculiar" velocities which
distort the uniform Hubble expansion.

3) In summary: The large-scale structure in the distribution of
galaxies is thought to arise from the gravitational instability
of small fluctuations in the initial density field of the
Universe. A key test of this hypothesis is that forming
superclusters of galaxies should generate a systematic infall of
other galaxies. This would be evident in the pattern of
recessional velocities, causing an anisotropy in the inferred
spatial clustering of galaxies. The authors report a precise
measurement of this clustering, using the redshifts of more than
141,000 galaxies from the two-degree-field (2dF) galaxy redshift
survey. The authors suggest that their results, combined with the
anisotropy of the cosmic microwave background, favor a low-
density Universe

References (abridged):
.
1. Hubble, E. P. The distribution of extra-galactic nebulae.
Astrophys. J. 79, 8-76 (1934)

2. Kirshner, R. P., Oemler, A., Schechter, P. L. & Shectman, S.
A. A million cubic megaparsec void in Bootes. Astrophys. J. 248,
L57-L60 (1981)

3. Davis, M. & Peebles, P. J. E. A survey of galaxy redshifts. V-
The two-point position and velocity correlations. Astrophys. J.
267, 465-482 (1983)

4. Bean, A. J., Ellis, R. S., Shanks, T., Efstathiou, G. &
Peterson, B. A. A complete galaxy redshift sample. I-The peculiar
velocities between galaxy pairs and the mean mass density of the
Universe. Mon. Not. R. Astron. Soc. 205, 605-624 (1983)

5. de Lapparent, V., Geller, M. J. & Huchra, J. P. A slice of the
universe. Astrophys. J. 302, L1-L5 (1986)

Nature 2001 410:169

Related Background:

ON GALAXY CLUSTERS AND THE HYDROGEN SPECTRUM

Robert Braun (Netherlands Foundation for Research in Astronomy,
NL) discusses galaxy clusters, the author making the following
points:

1) Hydrogen is the most abundant element in the Universe,
accounting for some 70% of the total mass in baryons (particles
such as protons and neutrons that experience the strong nuclear
force). In addition to being ubiquitous, the hydrogen atom acts
as an electric and magnetic dipole, giving rise to radiation
interactions that have enormous diagnostic value in observational
astronomy.

2) The electric dipole of hydrogen has long been exploited in
astronomy. Since the recombination spectrum of hydrogen was first
calculated about 100 years ago (1,2), this radiation has been
used to study emissions from energized regions and absorption by
quiescent regions at ever greater distances. The emission line
strength is directly proportional to the number of ionizing
photons. Strongly irradiated regions, such as the central regions
of quasars, can thus be detected even at extremely large
distances. The current record holder is at a redshift of 6.28
(3). Light reaching us from this distance has traveled for about
95% of the age of the universe.

3) The magnetic dipole properties of hydrogen give rise to a much
more subtle interaction. The tiny energy difference between
parallel and antiparallel spins of the proton-electron system of
atomic hydrogen corresponds to a radio photon with a wavelength
at rest of 21.12 cm. Van de Hulst predicted in 1945 that this
transition might lead to an observable phenomenon (4). Soon after
this prediction, the emission from atomic hydrogen clouds was
detected in our own galaxy, the Milky Way (5). 50 years after
this first detection, Zwaan et al. report the detection of the
21-cm emission of a galaxy comparable to our own at a redshift of
0.2. At twice the distance of the previous record, this
corresponds to a light travel time of about 20% of the age of the
universe.

4) The newly detected galaxy belongs to a massive galaxy cluster
named Abell 2218. This cluster has received considerable
attention because its large concentration of mass acts as a
gravitational lens that strongly bends the light from more
distant galaxies behind it. Abell 2218 is one of the densest
concentrations of galaxies in the local universe, containing many
hundreds of galaxies within the central 3 x 10^(6) light years.
For comparison, the Local Group of galaxies to which the Milky
Way belongs contains only three major galaxies within the same
radius.

References (abridged):

1. J. J. Balmer, Ann. Phys. Chem. 25, 80 (1885).

2. T. Lyman, Astrophys. J. 23, 18 (1906).

3. X. Fan et al., in preparation; available at
http://xxx.lanl.gov/abs/astro-ph/0108063

4. H. C. van de Hulst, Ned. Tijdsch. Natuurk. 11, 210 (1945).

5. H. I. Ewen, E. M. Purcell, Nature 168, 356 (1951).

Science 2001 293:1781

Related Background:

GALAXY CLUSTERS AND SUPERNOVA HEATING IN THE EARLY UNIVERSE

Contemporary cosmology distinguishes two kinds of matter,
"ordinary matter" and "dark matter". In general, a baryon is a
nuclear particle (e.g., a proton) built from 3 quarks
(fundamental particles that combine to make up protons, neutrons,
and mesons), and so-called "ordinary matter" is baryonic. In this
context, the term "dark matter" refers to material whose presence
can be inferred from its effects on the motions of stars and
galaxies, but which cannot be seen directly because it emits
little or no radiation. It is believed that as much as 90 percent
of the mass in the Universe may exist as some form or dark
matter, although the proposed percentage of dark matter varies
widely with different cosmological models.

The term "supernova" refers to a class of violently exploding
stars whose luminosity after eruption suddenly increases millions
or billions of times its normal level, the supernova explosion a
cataclysmic event associated with the essential end of the active
(energy-generating) life of the star.

G.M Voit and G.L. Bryan (Space Telescope Science Institute, US)
discuss galaxy clusters, the authors making the following points:

1) Clusters of galaxies are believed to contain approximately 10
times as much dark matter as baryonic matter. The dark component
therefore dominates the gravitational potential of a cluster, and
the baryons confined by this potential radiate x-rays with a
luminosity that depends mainly on the gas density in the core of
the cluster.

2) Predictions of the properties of these x-rays based on models
of cluster formation do not, however, agree with observations. If
the models ignore the condensation of cooling gas into stars and
also ignore feedback from the associated supernovae, the models
overestimate the x-ray luminosity because the simulated density
of the core gas is too high. An early episode of uniformly
distributed supernova feedback could rectify this by heating the
uncondensed gas and therefore making it more difficult for the
gas to compress into the core. But such a process seems to
require an implausibly large number of supernovae.

3) The authors demonstrate how radiative cooling of intergalactic
gas and subsequent supernova heating combine to eliminate highly-
compressible low-entropy gas from the intracluster medium. This
brings the galaxy cluster core entropy and x-ray luminosities of
clusters into agreement with observations in a way that depends
little on the efficiency of supernova heating in the early
Universe.

Nature 2001 414:425

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6. GALAXY ENCOUNTERS

A FOSSIL RECORD OF GALAXY ENCOUNTERS

D. Elbaz1 and C.J. Cesarsky (Service d'Astrophysique/CEA, FR)
discuss galaxy encounters, the authors making the following
points:

1) At this very moment, we are receiving light from stars born
throughout the lifetime of the Universe. Much of this light is in
the form of a diffuse background about 5% as bright as the cosmic
microwave background (CMB), a signature of the Big Bang. The
Cosmic Background Explorer (COBE) satellite -- which measured the
residual temperature of the Big Bang as well as the first
fluctuations of density when the Universe was only 300,000 years
old, the famous seeds of galaxy formation -- also permitted the
first detection of a diffuse background due to incipient galaxies
emitting light at wavelengths of 100 to 1000 microns, the cosmic
infrared background (CIRB). The way galaxies evolve from these
seeds to present-day galaxies like our own remains a mystery, and
we do not yet know with certainty the details of the cosmic
bookkeeping, the global evolution of the total energy emitted by
stars and galaxies.

2) The CIRB is a record of a large fraction of the emission of
light by stars and galaxies over cosmic history. If galaxies
formed through hierarchical merging, as predicted by current
models, then distant galaxies may only represent the small
precursors of mature galaxies like the Milky Way and galaxies in
its neighborhood, and galaxy formation is a continuous process.
Hence, the question "How did galaxies form?" may be restated as
"When did most of the stars form in galaxies?" And another
question arises: "Is there any connection between the development
of large-scale structures and star formation within galaxies?"
The information brought about by the CIRB and by the studies
attempting to determine its origin sheds some new light on these
questions.

3) In summary: The cosmic infrared background (CIRB) is a record
of a large fraction of the emission of light by stars and
galaxies over time. The bulk of this emission has been resolved
by the Infrared Space Observatory camera. The dominant
contributors are bright starburst galaxies with redshift z ~ 0.8;
that is, in the same redshift range as the active galactic nuclei
responsible for the bulk of the x-ray background. At the longest
wavelengths, sources of redshift z >= 2 tend to dominate the
CIRB. It appears that the majority of present-day stars have been
formed in dusty starbursts triggered by galaxy-galaxy
interactions and the buildup of large-scale structures.

References (abridged):

1. G. Hasinger, et al., Astron. Astrophys. 329, 482 (1998)

2. P. Rosati, et al., Astrophys. J. 566, 667 (2002)

3. N. Brandt, et al., Astron. J. 122, 2810 (2001)

4. G. Hasinger, et al., ESO Messenger 108, 11 (2002) .

5. P. Madau and L. Pozzetti, Mon. Not. R. Astron. Soc. 312, L9
(2000)

Science 2003 300:270

Related Background:

SIGNS OF GALACTIC CANNIBALISM

A. Helmi (Max Planck Institut f�r Astrophysik Garching bei
M�nchen, DE) discusses galactic cannibalism, the author making
the following points:

1) Just as paleontologists hunt for fossils to reconstruct the
evolution of life on Earth, so astronomers hunt for ancient
streams of galactic debris to reconstruct the formation history
of galaxies. Ibata et al (1)  report the discovery of a giant
stream-like structure in the Andromeda galaxy. Andromeda is a
large spiral galaxy like the Milky Way, and is one of our nearest
and largest galactic neighbors. Ibata et al. propose that strong
interactions between Andromeda and its smaller companion galaxies
led to the formation of this stream of stars. This kind of
"fossil" demonstrates the importance of gravitational
interactions in building and shaping mature galaxies, and may
further our understanding of the unseen "dark matter", which
constitutes 90% of the mass of the Universe.

2) Galaxies contain most of the stars in the Universe, yet how
they form and evolve remains largely mysterious. Most accepted
theories propose that they are the result of mergers and
accretion of smaller systems(2). This idea -- the hierarchical
build-up of structure in the Universe -- has gained substantial
support in the past ten years, mostly from observations of
distant galaxies caught in the act of forming new systems(3). Yet
present-day galaxies are expected to contain ancient structures
left over from the mergers that led to their formation. This sort
of fossil signature should be visible in neighboring galaxies.

3) Theories of galaxy formation suggest that spiral galaxies are
cannibals, feeding on any smaller systems that fall within their
grasp. In the sky, spiral galaxies such as the Milky Way look
surprisingly flat, with most of the stars concentrated in the
thin galactic disk formed by the spiral arms. Numerical
simulations of galaxy mergers predict that large spiral galaxies
should also have a spherical "halo" of diffuse matter stripped
from the smaller galaxies they have digested. This galactic halo
contains both stars (the stellar halo) and dark matter (the dark
halo). When a satellite galaxy is torn apart by a giant galaxy,
all that remains of the former satellite are trails of stars in
the resulting halo.

References (abridged):

1. Ibata, R., Irwin, M., Lewis, G., Ferguson, A. M. N. & Tanvir,
N. Nature 412, 49-52 (2001)

2. White, S. D. M. & Rees, M. J. Mon. Not. R. Astron. Soc 183,
341-358 (1978)

3. van den Bergh, S. et al. Astron. J 112, 359-368 (1996)

4. Ivezic, Z. et al. Astron. J. 120, 963-977 (2000)

5. Helmi, A., White, S. D. M., de Zeeuw P. T. & Zhao, H. S.
Nature 402, 53-55 (1999)

Nature 2001 412:25

Related Background:

GALACTIC CANNIBALISM

A "galactic halo", such as that associated with our own Galaxy,
is a spheroidal distribution of old stars and globular clusters
of old stars surrounding the galaxy. In the case of our own
Galaxy, the galactic halo has a radius of approximately 50,000
light years.

R. Ibata et al (Strasbourg Observatory, FR) discuss galactic
cannibalism, the authors making the following points:

1) Within the framework of the current theory of hierarchical
galaxy structure formation, large spiral galaxies like the Milky
Way or Andromeda arose from the merger of many small galaxies and
protogalaxies. Later in their evolution, spiral galaxies became
the dominant component in such mergers, cannibalizing smaller
systems that fell within their sphere of influence.

2) Recent observations have revealed streams of gas and stars in
the halo of the Milky Way that are the debris from interactions
between our Galaxy and some of its dwarf companion galaxies,
e.g., the Sagittarius dwarf galaxy and the Magellanic clouds.
Analysis of the material has demonstrated that much of the
Galactic halo consists of cannibalized satellite galaxies, and
that dark matter is distributed nearly spherically in the Milky
Way. It remains unclear, however, whether cannibalized
substructures are as common in the haloes of galaxies as
predicted by galaxy formation theory.

3) The authors report the discovery of a giant stream of metal-
rich stars within the halo of the nearest large galaxy, M31 (the
Andromeda galaxy). The source of this stream could be the dwarf
galaxies M32 and NGC205, which are close companions of M31 and
which may have lost a substantial number of stars as a
consequence of tidal interactions. The authors suggest these
results demonstrate that the epoch of galaxy building still
continues, albeit at a modest rate, and that tidal streams may be
a generic feature of galaxy haloes.

Nature 2001 412:49

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7. GALAXIES AND DARK MATTER

DIRECT DETECTION OF GALACTIC HALO DARK MATTER

B.R. Oppenheimer et al (University of California Berkeley, US)
discuss galactic halo dark matter, the authors making the
following points:

1) Dark matter in the spherical halo of the Milky Way galaxy has
been inferred because the gravitational field due to the known
distribution of luminous matter, primarily stars, cannot explain
the observed rotational characteristics of the galaxy's spiral
disk. A substantial portion of this unseen matter may be old,
very cool white dwarf stars (1-4). A white dwarf is the extremely
dense end-state in the evolution of stars with masses less than
about eight times the mass of the sun (M). Once a star becomes a
white dwarf, it no longer produces energy through nuclear fusion,
and therefore cools and fades.

2) The first four examples of ultracool white dwarfs, whose
temperatures are below 4000 K, were identified only in the past 2
years (2,3,5), principally because they were too faint to detect
in previous surveys. They have spectral energy distributions that
differ dramatically from those of the hotter white dwarfs,
consistent with white dwarf atmosphere models. The difference is
due to the formation of H2 molecules in white dwarf atmospheres
with effective temperatures below ~4500 K. Although H2 molecules
are symmetric and thus have no dipole moment, in the high
densities of white dwarf atmospheres, collisions between the
molecules are common. These collisions induce momentary dipole
moments, which produce opacity at wavelengths longer than 0.6
microns.

3) In summary: The Milky Way galaxy contains a large, spherical
component which is believed to harbor a substantial amount of
unseen matter. Recent observations indirectly suggest that as
much as half of this "dark matter" may be in the form of old,
very cool white dwarfs, the remnants of an ancient population of
stars as old as the galaxy itself. The authors conducted a survey
to find faint, cool white dwarfs with large space velocities,
indicative of their membership in the galaxy's spherical halo
component. The survey reveals a substantial, directly observed
population of old white dwarfs, too faint to be seen in previous
surveys. This newly discovered population accounts for at least 2
percent of the halo dark matter. It provides a natural
explanation for the indirect observations, and represents a
direct detection of galactic halo dark matter.

References (abridged):

1. C. Alcock, et al., Astrophys. J. 542, 281 (2000)

2. S. T. Hodgkin, et al., Nature 403, 57 (2000)

3. R. Ibata, M. J. Irwin, O. Bienaym‚, R. Scholz, J. Guibert,
Astrophys. J. Lett. 532, L41 (2000)

4. G. Chabrier, Astrophys. J. Lett. 513, L103 (1999)

5. H. C. Harris, et al., Astrophys. J. 524, 1000 (1999)

Science 2001 292:698

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8. ACTIVE GALACTIC NUCLEI

Some galaxies are known to have very active central regions from
which enormous amounts of energy are emitted each second, and it
is believed that these "active galactic nuclei" are probably
powered by accretion of matter into a supermassive black hole of
10^(6) to 10^(9) solar-masses. Astronomers have recently
discovered that many active galactic nuclei eject clouds of
ionized gas with velocities of up to 10 percent of the speed of
light over a wide range of angles, in contrast to the previously
known collimated jets. These mass outflows are considered to be
intriguing because they provide information about the dynamical
forces (such as radiation and wind pressure) near an active
supermassive black hole.

PLASMA JETS IN ACTIVE GALACTIC NUCLEI

A.P. Lobanov and J.A. Zensus (Max Planck Institute for Radio
Astronomy Bonn, DE) discuss plasma jets in active galactic
nuclei, the authors making the following points:

1) One of the most intriguing features observed in active
galactic nuclei is highly collimated and relativistic plasma
outflows (jets) that originate in the immediate vicinity of the
center of activity and propagate at distances of up to several
megaparsecs (1 parsec = 3.26 light years). Observations of jets
in active galactic nuclei probe the behavior of extremely
relativistic matter in the Universe and provide a unique and
remote "laboratory" for studying the most powerful cosmic
phenomena such as supermassive black holes and extragalactic
accretion disks.

2) The quasar 3C273 is one of the closest and most luminous and
best studied active galactic nuclei, with a prominent
relativistic outflow observed in the x-ray, optical, and radio
wave bands. The relativistic jet observed in this quasar is one-
sided, with no signs of emission on the counterjet side at
dynamic ranges of up to 16,000:1. This is evidence for strong
relativistic boosting in an intrinsically double-sided outflow
powered by an accretion disk around a black hole. The enhanced
emission features (jet components) identified in the jet on
scales of up to approximately 20 milli-arc seconds are moving at
apparent speeds exceeding the speed of light by factors of 5 to
8. These jet components may result from the flares observed in
this quasar in the optical and radio wavelengths and also reflect
the precession of the jet axis. The structure and kinematics of
such outflows are typically explained in terms of shock waves and
Kelvin-Helmholtz instability.

Science 2001 294:128

Related Background:

ON ACTIVE GALACTIC NUCLEI

Andrew C. Fabian (Cambridge University, UK) presents a short
account of current views in this field, the author making the
following points:

1) Active galactic nuclei involve the most powerful steady
sources of luminosity in the Universe. They range from the nuclei
of some nearby galaxies emitting approximately 10^(40) *erg/sec
to distant *quasars emitting more than 10^(47) erg/sec. The
emission is spread widely across the electromagnetic spectrum,
often peaking in the ultraviolet, but with significant luminosity
in the x-ray and infrared bands. The emission is spatially
unresolved except in the radio band, where there is sometimes
evidence for collimated outflows at *relativistic speeds. The
power output of active galactic nuclei are often variable on time
scales of years and sometimes on time scales of days, hours, or
even minutes.

2) An upper limit to the dimensions of active galactic nuclei can
be estimated from theoretical considerations. The principle of
causality (in general, in this context, that an effect cannot
precede its cause) implies that the variation period of an object
whose radiation emission varies in time must be greater than the
time for light to cross that object. Observed emission
variability, therefore, provides an upper bound to the diameter
of an active galactic nucleus.

3) High luminosities imply high masses whose gravity can combat
the radiation pressure that would otherwise blow the object apart
(in other words, the luminosity must be less than the *Eddington
limit). Active galactic nuclei therefore are of very high mass
density, and considering their apparent relatively small
dimensions and high mass density, it has long been assumed that
each active galactic nucleus consists of a massive black hole, of
approximately 10^(8) solar-mass or more, the black-hole mass
accreting gas and dust at the center of the galaxy. The
gravitational energy liberated during accretion onto a black hole
is estimated to be approximately 10 percent of the rest mass
energy of that accreting matter, and is the most efficient mass-
energy conversion process known involving normal matter. The
efficiency is at least an order of magnitude greater than that of
stellar nuclear burning, which releases at most 0.7 percent of
mass-energy.

3) The current view is that the accreting matter of the black
hole of an active galactic nucleus probably has some angular
momentum, which causes the accreting matter to orbit the black
hole and, through dissipation of energy, flatten to form a disk
within which *magnetic viscosity transfers the angular momentum
outward and the mass inward. Unless the accretion rate is either
high or very low, it is likely that the gravitational energy
liberated is radiated locally, much of it as thermal radiation
from the surface of the disk, peaking in the UV as expected. Some
energy, however, is probably stored temporarily in magnetic
fields before being released in flares, which make the x-ray
emission particularly variable.

Proc. Nat. Acad. Sci. 1999 96:4749

Notes:

*erg: The work done by a force of 1 dyne acting through a
distance of 1 centimeter. 1 joule = 10^(7) erg. 1 kilocal = 4.2 x
10^(10) erg.

*quasars: (quasi-stellar objects) Extremely luminous sources
radiating energy over the entire spectrum from x-rays to radio
waves, and which are apparently among oldest and most distant
objects in the universe.

*relativistic speeds: In general, speeds approaching the speed of
light. At such velocities, the mass of an object becomes
significantly greater than its rest mass.

*Eddington limit: The theoretical upper limit to the luminosity
of a star of given mass, at which limit the outward force of
radiation just balances the inward force of gravity. Stars with a
greater luminosity would be blown apart by their own radiation.
Named after A.S. Eddington (1882-1944).

*magnetic viscosity: In this context (plasma physics), the term
"magnetic viscosity" refers to an effect, possessed by a magnetic
field in the absence of sizable mechanical forces or electric
fields, of damping motions of a conducting fluid perpendicular to
the field, the effect similar to the effect of ordinary
viscosity. (The "conducting fluid" in this context is the
"plasma" of ionized gases.)

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