**********
HUBBLE RULES OUT A LEADING EXPLANATION FOR DARK MATTER, 16 November 1994
Two teams of astronomers, working independently with NASA's Hubble
Space Telescope (HST), have ruled out the possibility that red dwarf
stars constitute the invisible matter, called dark matter, believed to
account for more than 90 percent of the mass of the universe.
Until now, the dim, small stars were considered ideal candidates for
dark matter. Whatever dark matter is, its gravitational pull
ultimately will determine whether the universe will expand forever or
will someday collapse.
"Our results increase the mystery of the missing mass. They rule out a
popular but conservative interpretation of dark matter," said Dr. John
Bahcall, professor of natural science at the Institute of Advanced
Study, Princeton, NJ, and leader of one of the teams.
The group, led by Bahcall and Andrew Gould of Ohio State University,
Columbus, OH, (formerly of the Institute for Advanced Study), showed
that faint red dwarf stars, which were thought to be abundant, actually
are sparse in the Earth's home galaxy, and in the universe by
inference.
The team, led by Dr. Francesco Paresce of the Space Telescope Science
Institute in Baltimore, MD, and the European Space Agency, determined
that the faint red stars rarely form and that there is a cutoff point
below which nature does not make this type of dim, low-mass star.
The space telescope observations involved accurately counting stars and
gauging their brightness. The observations overturn several decades of
conjecture, theory and observation about the typical mass and abundance
of the smallest stars in the universe.
PREVIOUS GROUND-BASED RESULTS INCONCLUSIVE
In our own stellar neighborhood, there are almost as many red dwarfs as
there are all other types of stars put together. The general trend
throughout our galaxy is that small stars are more plentiful than
larger stars, just as there are more pebbles on the beach than rocks.
This led many astronomers to believe that they were only seeing the tip
of the iceberg and that many more extremely faint red dwarf stars were
at the limits of detection with ground-based instruments.
According to stellar evolution theory, stars as small as eight percent
of the mass of the Sun are still capable of shining by nuclear fusion
processes.
Over the past two decades, theoreticians have suggested that the lowest
mass stars also should be the most prevalent and therefore might
provide a solution for dark matter. This seemed to be supported by
previous observations with ground-based telescopes that hinted at an
unexpected abundance of what appeared to be red stars at the faintest
detection levels achievable from the ground.
However, these prior observations were uncertain because the light from
these faint objects is blurred slightly by Earth's turbulent
atmosphere. This makes the red stars appear indistinguishable from the
far more distant, diffuse-looking galaxies.
PINNING DOWN THE LONG-SOUGHT HALO POPULATION
Hubble's capabilities made it possible for the team of astronomers led
by Bahcall and Gould to observe red stars that are 100 times dimmer
than those detectable from the ground -- a level where stars can be
distinguished easily from galaxies. Hubble Space Telescope's extremely
high resolution also can separate faint stars from the much more
numerous galaxies by resolving the stars as distinct points of light,
as opposed to the "fuzzy" extended signature of a remote galaxy.
Bahcall and Gould, with their colleagues Chris Flynn and Sophia
Kirhakos (also of the Institute for Advanced Study, Princeton) used
images of random areas in the sky taken with the HST Wide Field
Planetary Camera 2 while the telescope was performing scheduled
observations with other instruments. By simply counting the number of
faint stars in the areas observed by HST, the scientists demonstrated
that the Milky Way has relatively few faint red stars.
The HST observations show that dim red stars make up no more than six
percent of the mass in the halo of the Galaxy, and no more than 15
percent of the mass of the Milky Way's disk. The Galactic halo is a
vast spherical region that envelops the Milky Way's spiral disk of
stars, of which Earth's Sun is one inhabitant.
FAINT RED STARS MISSING FROM A GLOBULAR CLUSTER
By coincidence, Paresce pursued the search for faint red dwarfs after
his curiosity was piqued by an HST image taken near the core of the
globular cluster NGC 6397. He was surprised to see that the inner
region was so devoid of stars, he could see right through the cluster
to far more distant background galaxies. Computer simulations based on
models of stellar population predicted the field should be saturated
with dim stars-- but it wasn't.
HST's sensitivity and resolution allowed Paresce, and co-investigators
Guido De Marchi (Space Telescope Science Institute and the University
of Firenze, Italy), and Martino Romaniello (University of Pisa, Italy)
to conduct the most complete study to date of the population of the
cluster (globular clusters are ancient, pristine laboratories for
studying stellar evolution). To Paresce's surprise, he found that
stars 1/5 the mass of our Sun are very abundant -- there are about 100
stars this size for every single star the mass of our Sun -- but that
stars below that range are rare. "The very small stars simply don't
exist," he said.
A star is born as a result of the gravitational collapse of a cloud of
interstellar gas and dust. This contraction stops when the infalling
gas is hot and dense enough to trigger nuclear fusion, causing the star
to glow and radiate energy. "There must be a mass limit below which the
material is unstable and cannot make stars," Paresce emphasizes.
"Apparently, nature breaks things off below this threshold."
Paresce has considered the possibility that very low-mass stars formed
long ago but were thrown out of the cluster due to interactions with
more massive stars within the cluster, or during passage through the
plane of our Galaxy. This process would presumably be common among the
approximately 150 globular clusters that orbit the Milky Way. However,
the cast-off stars would be expected to be found in the Milky Way's
halo, and Bahcall's HST results don't support this explanation.
THE SEARCH FOR DARK MATTER
The HST findings are the latest contribution to a series of recent,
intriguing astronomical observations that are struggling to pin down
the elusive truth behind the universe's "missing mass."
Models describing the origin of helium and other light elements during
the birth of the universe, or "Big Bang," predict that less than 5% of
the universe is made up of "normal stuff," such as neutrons and
protons. This means more than 90% of the universe must be some unknown
material that does not emit any radiation that can be detected by
current instrumentation. Candidates for dark matter include black
holes, neutron stars and a variety of exotic elementary particles.
Within the past year, astronomers have uncovered indirect evidence for
a dark matter candidate called a MACHO (MAssive Compact Halo Objects).
These previous observations detected several instances of an invisible
object that happens to lie along the line of sight to an extragalactic
star. When the intervening object is briefly aligned between Earth and
a distant star, it amplifies, or gravitationally lenses, the light from
the distant star.
The new HST finding shows that faint red stars are not abundant enough
to explain the gravitational lensing events attributed to MACHOs.
Bahcall cautions, however, that his results do not rule out other halo
objects that could be smaller than the red stars such as brown dwarfs
-- objects not massive enough to burn hydrogen and shine in visible
light.
Additional circumstantial evidence for dark matter in the halo of our
galaxy has been inferred from its gravitational influence on the
motions of stars within the Milky Way's disk. Recently, this notion
was further supported by ground-based observations, made by Peggy
Sachett of the Institute for Advanced Study, that show a faint glow of
light around a neighboring spiral galaxy that is the shape expected for
a halo composed of dark matter. This could either be light from the
dark matter itself or stars that trace the presence of the galaxy's
dark matter.
The reality of dark matter also has been inferred from the motions of
galaxies in clusters, the properties of high-temperature gas located in
clusters of galaxies and from the relative amounts of light elements
and isotopes produced in the Big Bang.
The ultimate fate of the universe will be determined by the amount of
dark matter present. Astronomers have calculated that the amount of
matter -- planets, stars and galaxies -- observed in the universe
cannot exert enough gravitational pull to stop the expansion which
began with the Big Bang. Therefore, if the universe contains less than
a critical density of matter, it will continue expanding forever, but
if enough of the mysterious dark matter exists, the combined
gravitational pull someday will cause the universe to stop expanding
and eventually collapse.
Bahcall stresses, "The dark matter problem remains one of the
fundamental puzzles in physics and astronomy. Our results only sharpen
the question of what is the dark matter."
Bahcall's results appeared in the November 1, 1994 issue of the
Astrophysical Journal. Paresce's paper will appear in the February 10,
1995, issue of the Astrophysical Journal.
The Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, Inc. (AURA) for NASA, under
contract with the Goddard Space Flight Center, Greenbelt, MD. The
Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- T o d d E. V a n H o o s e a r -
``'''vanhoose@lalaland.cl.msu.edu - vanhoose@msu.edu - vanhoose@lalaland.cl.msu.edu
(._.) Michigan State University - East Lansing, MI USA
(_) Computer Laboratory - Department of Communication
`---' <A HREF="http://lalaland.cl.msu.edu/~vanhoose/">My Home Page</A>
PGP-aware: finger vanhoose@lalaland.cl.msu.edu for my public key
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~