Dwarf Galaxies Need Dark Matter Too, Astronomers
ScienceDaily (Oct. 28, 2007) —
Stars in dwarf spheroidal galaxies behave in a way that suggests
the galaxies are utterly dominated by dark matter, University of
Michigan astronomers have found.
Carina Nebula. A small portion of the
rough-and-tumble neighborhood of swirling dust and gas near
one of the most massive and eruptive stars in our galaxy is
seen in this image. This close-up view shows only a three
light-year-wide portion of the entire Carina Nebula, which
has a diameter of over 200 light-years. Located 8,000
light-years from Earth, the nebula can be seen in the
southern sky with the naked eye. (Credit: NASA and the
Hubble Heritage Team)
Astronomy professor Mario Mateo and post-doctoral
researcher Matthew Walker measured the velocity of 6,804 stars
in seven dwarf satellite galaxies of the Milky Way: Carina,
Draco, Fornax, Leo I, Leo II, Sculptor and Sextans. They found
that, contrary to what Newton's law of gravity predicts, stars
in these galaxies do not move slower the farther they are from
their galaxy's core.
"These galaxies show a problem right from the
center," Mateo said. "The velocity doesn't get smaller. It just
stays the same, which is eerie."
Astronomers already know stars in spiral galaxies
behave in a similar way. This research dramatically increases
the available information about smaller galaxies, making it
possible to confirm that the distribution of light and stars in
them is not the same as the distribution of mass.
"We have more than doubled the amount of data
having to do with these galaxies, and that allows us to study
them in an unprecedented manner. Our research shows that dwarf
galaxies are utterly dominated by dark matter, so long as
Newtonian gravity adequately describes these systems," Walker
said. Walker received his doctorate from U-M earlier this year
and currently has a post-doctoral position at the University of
Cambridge in the United Kingdom.
Dark matter is a substance astronomers have not
directly observed, but they deduce it exists because they detect
its gravitational effects on visible matter. Based on these
measurements, the prevailing theory in astronomy and cosmology
is that the visible parts of the universe make up only a
fraction of its total matter and energy.
The planet Neptune was once "dark matter," Mateo
said. Before the term was even coined, astronomers predicted its
existence based on an anomaly in the orbit of Neptune's neighbor
Uranus. They knew just where to look for Neptune.
For the past quarter century, astronomers have
been looking for the Neptune of the universe, so to speak. Dark
matter could take the form of dwarf stars and planets,
elementary particles including neutrinos, or hypothetical and
as-yet undetected particles that don't interact with visible
light or other parts of the electromagnetic spectrum.
Dark matter is believed to hold galaxies
together. The gravitational force of the visible matter is not
considered strong enough to prevent stars from escaping. Other
theories exist to explain these discrepancies, though. For
example, Modified Newtonian Dynamics, Mateo said, proposes that
gravitational forces become stronger when accelerations are very
weak. While their results align with current dark matter models,
Mateo and Walker say they also bolster this less-popular
"These dwarf galaxies are not much to look at,"
Mateo continued, "but they may really alter our fundamental
views on the nature of dark matter and, perhaps, even gravity."
Walker will present a paper on these findings on
Oct. 30 at the Magellan Science Meeting in Cambridge, Mass. The
paper he will present is Velocity Dispersion Profiles of Seven
Dwarf Spheroidal Galaxies. It was published in the Sept. 20
edition of Astrophysical Journal Letters.
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