Six Cluster Collisions, with Dark-Matter Maps (Hubble and Chandra — Annotated)
The clusters shown here are, from left to right and top to bottom:
MACS J0416.1-2403, MACS J0152.5-2852, MACS J0717.5+3745, Abell 370,
Abell 2744, and ZwCl 1358+62.
Science Credit: NASA, ESA,
D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland;
University of Edinburgh, UK), R. Massey (Durham University, UK), T.
Kitching (University College London, UK), and A. Taylor and E. Tittley
(University of Edinburgh, UK). Image Credit: NASA, ESA, STScI, and CXC.
Astronomers using observations from NASA's Hubble Space Telescope and
Chandra X-ray Observatory have found that dark matter does not slow
down when colliding with each other. This means that it interacts with
itself even less than previously thought. Researchers say this finding
narrows down the options for what this mysterious substance might be.
Dark matter is a transparent form of matter that makes up most of the
mass in the universe. Because dark matter does not reflect, absorb, or
emit light, it can only be traced indirectly, such as by measuring how
it warps space through gravitational lensing, where the light from
distant sources is magnified and distorted by the gravitational effects
of dark matter.
The two space observatories were used to study how dark matter in
clusters of galaxies behaves when the clusters collide. Hubble was used
to map the post-collision distribution of stars and dark matter, which
was traced through its gravitational lensing effects on background
light. Chandra was used to see the X-ray emission from the colliding
gas. The results will be published in the journal Science on March 27.
"Dark matter is an enigma we have long sought to unravel," said John
Grunsfeld, assistant administrator of NASA's Science Mission Directorate
in Washington. "With the combined capabilities of these great
observatories, both in extended mission, we are ever closer to
understanding this cosmic phenomenon."
To learn more about dark matter, researchers can study it in a way
similar to experiments on visible matter — by watching what happens
when it bumps into celestial objects. An excellent natural laboratory
for this analysis can be found in collisions between galaxy clusters.
Galaxy clusters are made of three main ingredients: galaxies, clouds
of gas, and dark matter. During collisions, the clouds of gas
enveloping the galaxies crash into each other and slow down or stop.
The galaxies are much less affected by the drag from the gas and,
because of the huge gaps between the stars within them, do not have a
slowing effect on each other.
"We know how gas and galaxies react to these cosmic crashes and where
they emerge from the wreckage. Comparing how dark matter behaves can
help us to narrow down what it actually is," explained David Harvey of
the École Polytechnique Fédérale de Lausanne, Switzerland, lead author
of the new study.
Harvey and his team used data from Hubble and Chandra to study 72
large cluster collisions. The collisions happened at different times,
and are seen from different angles — some from the side, and others
head-on.
The team found that, like the galaxies, the dark matter continued
straight through the violent collisions without slowing down relative
to the galaxies. Because galaxies pass through unimpeded, if
astronomers observe a separation between the distribution of the
galaxies and the dark matter then they know it has slowed down. If the
dark matter does slow, it will drag and lie somewhere between the
galaxies and the gas, which tells researchers how much it has
interacted.
The leading theory is that dark matter particles spread throughout
the galaxy clusters do not frequently bump into each other. The reason
the dark matter doesn't slow down is because not only does it not
interact with visible particles, it also infrequently interacts with
other dark matter. The team has measured this "self-interaction" and
found it occurs even less frequently than previously thought.
"A previous study had seen similar behavior in the Bullet Cluster,"
said team member Richard Massey of Durham University, U.K. "But it's
difficult to interpret what you're seeing if you have just one example.
Each collision takes hundreds of millions of years, so in a human
lifetime we only get to see one freeze-frame from a single camera
angle. Now that we have studied so many more collisions, we can start
to piece together the full movie and better understand what is going
on."
By finding that dark matter interacts with itself even less than
previously thought, the team has successfully narrowed down the
properties of dark matter. Particle physics theorists now have a
smaller set of unknowns to work with when building their models.
"It is unclear how much we expect dark matter to interact with itself
because dark matter is already going against everything we know, said
Harvey. "We know from previous observations that it must interact with
itself reasonably weakly, however this study has now placed it below
that of two protons interacting with one another — which is one theory
for dark matter." Harvey said that the results suggest that dark matter
is unlikely to be only a kind of dark proton. If dark matter scattered
like protons do with one another (electrostatically) it would have been
detected. "This challenges the idea that there exists 'dark photons,'
the dark matter equivalent of photons," he said.
Dark matter could potentially have rich and complex properties, and
there are still several other types of interactions to study. These
latest results rule out interactions that create a strong frictional
force, causing dark matter to slow down during collisions. Other
possible interactions could make dark matter particles bounce off each
other like billiard balls, causing dark matter particles to be ejected
from the clouds by collisions or for dark matter blobs to change shape.
The team will be studying these next.
To further increase the number of collisions that can be studied, the
team is also looking to study collisions involving individual
galaxies, which are much more common.
"There are still several viable candidates for dark matter, so the
game is not over, but we are getting nearer to an answer," concludes
Harvey. "These 'astronomically large' particle colliders are finally
letting us gimpse the dark world all around us but just out of reach."
Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
Georgia Bladon
ESA/Hubble, Garching, Germany
011-44-7816-291261
gbladon@partner.eso.org
Richard Massey
Durham University, Durham, UK
011-44-7740-648080
r.j.massey@durham.ac.uk
David Harvey
EPFL, Lausanne, Switzerland;
University of Edinburgh, Edinburgh, UK
011-41-22-3792475
david.harvey@epfl.ch
Source: HubbleSite
