[tt] There's A Dark Matter 'Disk' In Our Own Milky Way Galaxy, Say Researchers

[Eugen Leitl]

http://www.scientificblogging.com/news_releases/theres_a_dark_matter_disk_in_our_own_milky_way_galaxy_say_researchers?

There's A Dark Matter 'Disk' In Our Own Milky Way Galaxy, Say Researchers

Submitted by News Staff on 16 September 2008 - 12:31am. Space

An international team of scientists predict that our galaxy, the Milky Way,
contains a disk of ‘dark matter’, in a paper published in Monthly Notices of
the Royal Astronomical Society.

Astronomers Dr. Justin Read, Professor George Lake and Oscar Agertz of the
University of Zurich, and Dr. Victor Debattista of the University of Central
Lancashire used the results of a supercomputer simulation to deduce the
presence of this disk. They explain how it could allow physicists to directly
detect and identify the nature of dark matter for the first time.

Unlike the familiar ‘normal’ matter that makes up stars, gas and dust, ‘dark’
matter is invisible but its presence can be inferred through its
gravitational influence on its surroundings. Physicists believe that it makes
up 22% of the mass of the Universe (compared with the 4% of normal matter and
74% comprising the mysterious ‘dark energy’). But, despite its pervasive
influence, no one is sure what dark matter consists of.

Prior to this work, it was thought that dark matter forms in roughly
spherical lumps called ‘halos’, one of which envelopes the Milky Way. But
this standard theory is based on supercomputer simulations that model the
gravitational influence of the dark matter alone. The new work includes the
gravitational influence of the stars and gas that also make up our galaxy.

Stars and gas are thought to have settled into disks very early on in the
life of the Universe and this affected how smaller dark matter halos formed.
The team’s results suggest that most lumps of dark matter in our locality
merged to form a halo around the Milky Way. But the largest lumps were
preferentially dragged towards the galactic disk and were then torn apart,
creating a disk of dark matter within the Galaxy.

“The dark disk only has about half of the density of the dark matter halo,
which is why no one has spotted it before,” said lead author Justin Read.
“However, despite its low density, if the disk exists it has dramatic
implications for the detection of dark matter here on Earth.”

The Earth and Sun move at some 220 kilometres per second along a nearly
circular orbit about the centre of our Galaxy. Since the dark matter halo
does not rotate, from an Earth-based perspective it feels as if we have a
‘wind’ of dark matter flowing towards us at great speed. By contrast, the
‘wind’ from the dark disk is much slower than from the halo because the disk
co-rotates with the Earth.

“It's like sitting in your car on the highway moving at a hundred kilometres
an hour”, said team member Dr Victor Debattista. “It feels like all of the
other cars are stationary because they are moving at the same speed.”

This abundance of low-speed dark matter particles could be a real boon for
researchers because they are more likely to excite a response in dark matter
detectors than fast-moving particles. “Current detectors cannot distinguish
these slow moving particles from other background ‘noise’,” said Prof. Laura
Baudis, a collaborator at the University of Zurich and one of the lead
investigators for the XENON direct detection experiment, which is located at
the Gran Sasso Underground Laboratory in Italy. “But the XENON100 detector
that we are turning on right now is much more sensitive. For many popular
dark matter particle candidates, it will be able to see something if it’s
there.”

This new research raises the exciting prospect that the dark disk – and dark
matter – could be directly detected in the very near future.