An intense quasar can, like a
flashlight, illuminate part of the surrounding cosmic web
The hidden tendrils
of dark matter that underlie the visible Universe may have been
traced out for the first time.
Cosmology theory predicts that galaxies are embedded in a cosmic
web of "stuff", most of which is dark matter.
Astronomers obtained the first direct images of a part of this
network, by exploiting the fact that a luminous object called a
quasar can act as a natural "cosmic flashlight".
Details of the work appear in the journal Nature.
The quasar illuminates a nearby gas cloud measuring two million
light-years across.
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In this case we were lucky that the
flashlight is pointing toward the nebula and making the gas
glow”
Sebastiano Cantalupo
University of California, Santa
Cruz
And the glowing gas appears to trace out
filaments of underlying dark matter.
The quasar, which lies 10 billion light-years away, shines light
in just the right direction to reveal the cold gas cloud.
For some years, cosmologists have been running computer
simulations of the structure of the universe to build the "standard
model of cosmology".
They use the cosmic microwave background, corresponding to
observations of the very earliest Universe that can be seen, and
recorded by instruments such as the Planck space observatory, as a
starting point.
Their calculations suggest that as the Universe grows and forms,
matter becomes clustered in filaments and nodes under the force of
gravity, like a giant cosmic web.
The new results from the 10-metre Keck telescope in Hawaii,
are reported by scientists from the University of California,
Santa Cruz and the Max Planck Institute for Astronomy in Heidelberg.
They are the first direct observations of cold gas decorating
such cosmic web filaments.
The observed portion of the cosmic
web (cyan) is about 2 million light-years across
The cosmic web suggested by the standard model is mainly made up
of mysterious "dark matter". Invisible in itself, dark matter still
exerts gravitational forces on visible light and ordinary matter
nearby.
Massive clumps of dark matter bend light that passes close by
through a process called gravitational lensing, and this had allowed
previous measurements of its distribution.
But it is difficult to use this method to see very distant dark
matter, and cold ordinary matter remains tricky to detect as well.
The glowing hydrogen illuminated by the distant quasar in these
new observations traces out an underlying filament of dark matter
that it is attracted to it by gravity, according to the researchers'
analysis.
"This is a new way to detect filaments. It seems that they have a
very bright quasar in a rare geometry," Prof Alexandre Refregier of
the ETH Zurich, who was not involved in the work, told BBC News.
"If indeed gravity is doing the work in an expanding Universe, we
expect to see a cosmic web and it is important to detect this cosmic
web structure."
In the dark
He added: "What is expected is that the dark matter dominates the
mass and forms these structures, and then the ordinary matter, the
gas, the stars and everything else trace the filaments and
structures that are defined by the dynamics of the dark matter."
"Filaments have been detected indirectly before using
gravitational lensing, which allows us to see the distribution of
the dark matter.
"Part of the ordinary matter has formed stars, which we can see,
but another component is the gas. If the gas is very hot it emits
X-rays and can be seen using X-ray telescopes. Other techniques to
detect cooler gas now include the method described here."
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Dark energy and dark matter mysteries
- Gravity acting across vast distances does not seem to
explain what astronomers see
- Galaxies, for example, should fly apart; some other mass
must be there holding them together
- Astrophysicists have thus postulated "dark matter" -
invisible to us but clearly acting on galactic scales
- At the greatest distances, the Universe's expansion is
accelerating
- Thus we have also "dark energy" which acts to drive the
expansion, in opposition to gravity
- The current theory holds that 68% of the Universe is
dark energy, 27% is dark matter, and just 5% the kind of
matter we know well
Sebastiano Cantalupo, lead author of the
article, and others have used the same method previously to look for
glowing gas around quasars, and
had seen dark galaxies.
"The dark galaxies are much denser and smaller parts of the
cosmic web. In this new image, we also see dark galaxies, in
addition to the much more diffuse and extended nebula," Dr
Cantalupo, from UCSC, explained.
"Some of this gas will fall into galaxies, but most of it will
remain diffuse and never form stars.
"The light from the quasar is like a flashlight beam, and in this
case we were lucky that the flashlight is pointing toward the nebula
and making the gas glow. We think this is part of a filament that
may be even more extended than this, but we only see the part of the
filament that is illuminated by the beamed emission from the
quasar."
While the observations support the cosmological simulations'
general picture of a cosmic web of filamentary structures, the
researchers' results suggest around 10 times more gas in the nebula
than predicted from typical computer simulations.
They postulate that this may simply be due to limitations in the
spatial resolution of the current models, or, more interestingly
perhaps, may be because the current grid-based models are missing
some aspect of the underlying physics of how galaxies form, evolve,
and interact with quasars.
"We now have very precise measurements of the amount of ordinary
matter and dark matter in the Universe," said Prof Refregier.
"We can only observe a fraction of the ordinary matter, so the
question is what form the remainder takes. These results may imply
that a lot of it is in the form detected here."
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