Two University of Toronto astronomers and a U.S. colleague have made the first measurements of the size and shape of massive dark matter halos that surround galaxies.
‘Our findings give us the clearest picture yet of a very mysterious
part of
our universe,’ says principal investigator Henk Hoekstra, a
post-doctoral
fellow at University of Toronto’s Canadian Institute for Theoretical
Astrophysics. ‘Using relatively simple physics, we can get our first
direct
glimpse of the size and shape of these halos which are more than fifty
times
more massive than the light-producing part of galaxies that we can
see.’ He
and his team present their findings on July 25 at the 25th General
Assembly
of the International Astronomical Union in Sydney, Australia.
Their research indicates that dark matter halos extend more than five
times
further than the visible stars in a galaxy, says Hoekstra. In the case
of
our Milky Way galaxy, he says, the halo extends to more than 500,000
light-years away and weighs approximately 880 billion times more than
the
sun. The findings also provide strong support for the popular ‘cold
dark
matter’ model of the universe.
Dark matter emits no light and, therefore, cannot be seen directly,
Hoekstra
explains. The only evidence for its existence comes from its
gravitational
pull on stars, gas and light rays. Dark matter is believed to account
for
approximately 25 per cent of the total mass in the universe, with the
rest
of the universe composed of normal matter (five per cent) and dark
energy
(70 per cent).
To date, most information about dark matter has come from measurements
of
the motion of gas and stars in the inner regions of galaxies. Other
important data have come from computer simulations of the formation of
the
universe’s structure. However, scientists can explain their findings
about
dark matter only if it is true that galaxies are surrounded by massive,
three-dimensional halos.
The majority of astronomers believe in the so-called cold dark matter
theory
of the universe, which suggests these halos are slightly flattened.
Hoekstra’s findings corroborate this. Using the relatively new
technique of
weak gravitational lensing which allows astronomers to study the size
and
shape of dark matter, the team measured the shapes of more than 1.5
million
distant galaxies using the Canada-France-Hawaii Telescope in Hawaii.
‘The
small changes in the shapes of the galaxies offered a strong
indication to
us that the halos are flattened, like a rubber ball compressed to half
its
size,’ Hoekstra says.
Their findings can also be applied to a larger scientific debate about
the
nature of the universe. Some scientists have developed theories about
the
universe using the assumption that dark matter does not exist and, as a
result, they have proposed changes to the law of gravity. However,
Hoekstra
is confident that his team’s findings will refute these theories.
The research was conducted with Professor Howard Yee of University of
Toronto’s Department of Astronomy and Astrophysics and Michael
Gladders, a
former University of Toronto graduate student now at the Observatories
of
the Carnegie Institution of Washington in Pasadena, Calif. It was
funded by
the Natural Sciences and Engineering Research Council of Canada and the
University of Toronto.
IMAGE:
Image Credit: Credits: NASA, Andrew Fruchter and the ERO Team [Sylvia
effect called ‘gravitational lensing’ to study the ‘dark matter’ halos
that
surround galaxies. They’ve found that the halos are up to five times
bigger
than the galaxies themselves.
Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)] (STScI)
