[tt] NS: Are dwarf galaxies disguised by dark matter decay?

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Are dwarf galaxies disguised by dark matter decay?
http://space.newscientist.com/article.ns?id=mg19826604.000&print=true
12 June 2008
Anil Ananthaswamy

IF YOU want to be eye-catching, bigger is usually better. Not so for
dwarf galaxies. It seems that if dwarfs have been fattened up by
decaying dark matter, they are harder to spot. This could explain a
decades-old puzzle over why we have spotted so few dwarfs circling
the Milky Way and nearby giants such as Andromeda (pictured).

Astronomers have so far found 20 or so dwarf galaxies in the
vicinity of the Milky Way. This number is about 20 times fewer than
is predicted by our best model to describe the large-scale structure
of the observed universe since the big bang.

According to this model, first to form after the big bang are small
clumps, or haloes, of dark matter. These then collide to form bigger
and bigger haloes, around which gas coalesces to form stars,
galaxies and clusters. This explains the formation of large galaxies
such as the Milky Way just fine, but it predicts far greater numbers
of dwarf galaxies than have been observed.

Some astrophysicists have argued that the number of dwarfs must have
become depleted somehow. For instance, supernovae might explode
early in the life of such galaxies, blowing away gas and preventing
new stars from forming. "But nobody has shown that any of these
processes alone can account for the disparity," says Fulvio Melia of
the University of Arizona in Tucson.

Melia and his colleague Majd Abdelqader suspected that dark matter
might have something to do with the disparity, so they took a
different tack. They built models in which dark matter was allowed
to decay into other particles.

To their surprise, they found that the deficit could be explained if
a few parts in 100,000 of the dark matter decayed over a period of
between 4 and 20 billion years. The products of the decay would
carry more kinetic energy than the dark matter, which would cause
them to move faster in larger orbits. This would force the dwarfs'
halo to expand.

This process wouldn't make much difference to larger galaxies like
the Milky Way with lots more dark matter, but in dwarf galaxies such
puffed-up haloes would have two effects. Firstly, the dark matter
halo would become less dense, giving it less of a gravitational tug
on the stars. This means the stars would travel more slowly around
the galaxy centre. With today's telescopes, only galaxies in which
stars are moving with velocities greater than 5 kilometres per
second can be detected. In Melia's simulation, the stars are moving
more slowly than this (www.arxiv.org/abs/0806.0602).

"There is another effect that would kick in too," says Melia. In
some cases, the dark matter halo expands so much that the gas in the
galaxy becomes very diffuse, preventing the formation of stars and
making the galaxies dimmer overall and so harder to spot.

Astrophysicist Gerry Gilmore of the University of Cambridge, whose
team recently found 11 dwarf galaxies around the Milky Way, says
that Melia's work is "one of the most interesting explanations" for
the missing dwarf galaxy problem.

Cosmology - Keep up with the latest ideas in our special report.

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Weblinks

Melia and Abdelqadar's paper
http://arxiv.org/abs/0806.0602
Fulvio Melia
http://www.physics.arizona.edu/~melia/