NASA's Hubble
Space Telescope has uncovered new evidence that galaxies are embedded in and
protected by halos of dark matter, the invisible form of matter that accounts
for most of the universe's mass.
Dark matter
is invisible and nobody even knows what it is, but it is evident by the fact
that galaxies hold together at all. Some unseen substance lurks in space —
concentrated in galaxies — and generated gravity in amounts well beyond the
visible matter.
Peering
into the tumultuous heart of the nearby Perseus
galaxy cluster (located 250 million light-years away), Hubble
discovered a large population of small galaxies that have remained intact while
larger galaxies around them are being ripped apart by the gravitational pull of
neighboring galaxies.
"We
were surprised to find so many dwarf galaxies in the core of this cluster that
were so smooth and round and had no evidence at all of any kind of
disturbance," said astronomer Christopher Conselice of the University of
Nottingham in England, and leader of the Hubble observations. "These dwarfs are
very old galaxies that have been in the cluster a long time. So if something
was going to disrupt them, it would have happened by now. They must be very,
very dark matter-dominated galaxies."
The dwarf
galaxies may have even higher amounts of dark
matter than spiral galaxies like our Milky Way.
"With these
results, we cannot say whether the dark-matter content of the dwarfs is higher
than in the Milky Way Galaxy," Conselice said. "Although, the fact that spiral
galaxies are destroyed in clusters, while the dwarfs are not, suggests that is
indeed the case."
The results
of the observations are detailed in the March 1 edition of the journal Monthly
Notices of the Royal Astronomical Society.
First
proposed about 80 years ago, dark matter is thought to be the "glue" that holds
galaxies together. Astronomers suggest that dark matter provides vital
"scaffolding" for the universe, forming a framework for the formation of
galaxies through gravitational attraction.
Previous
studies with Hubble and NASA's Chandra
X-ray Observatory found evidence of dark matter in entire clusters of
galaxies such as the Bullet Cluster. The new Hubble observations continue the
search for dark matter in individual galaxies.
Because
dark matter cannot be seen, astronomers detect its presence through indirect
evidence. The most common method is by measuring the velocities of individual
stars or groups of stars as they move randomly in the galaxy or as they rotate
around the galaxy. But the Perseus Cluster is too far away for telescopes to
resolve individual stars and measure their motions.
Instead,
Conselice and his team derived a new technique for uncovering dark matter in
these dwarf galaxies by determining the minimum mass the dwarfs must have to
protect them from being disrupted by the strong tidal pull of gravity from
larger galaxies.
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