- 4319 -
DARK MATTER - new
discoveries? Blobs of “Dark Matter”
have been observed down to a scale of 30,000 light-years across. Dark matter remains mysterious. We don’t yet have a definite idea of what
this cosmic “stuff” is made of, astronomers are learning more about its
distribution throughout the Universe.
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------------------ 4319 - DARK MATTER - new discoveries?
- Since we can’t see
dark matter directly, observers need to
use indirect methods to detect it. One way is through “gravitational lensing”.
Another is by looking for emissions from hydrogen gas associated with
small-scale dark matter structures in the Universe.
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- Astronomers used
the Atacama Large Millimeter Array in Chile to study a distant gravitational
lens system “MG J0414+534”. A massive
foreground galaxy is bending and distorting the light from a distant quasar
that lies some 11 billion light-years away.
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- There are actually
variations in the distribution of dark matter along the line of sight between
us and the quasar. The gravitational lens magnified the fluctuations and
analysis of the data allowed them to map the fluctuations down to a scale of
30,000 light-years.
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- What do the blobs
of dark matter mean? Throughout the
universe, dark matter is associated with massive galaxies and galaxy clusters.
However, small-scale clumps and distributions aren’t as well understood. So,
astronomers want to find ways to map the smaller concentrations of it.
-
- “Gravitational
lensing” provides one way to do that. In the case of “MG J0414+0534”, the
positions and shapes of the lensed quasar images look a little strange. They
don’t fit the model of gravitational lensing predicted when you plug in the
numbers for the galaxy and its associated dark matter component.
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- The fluctuations
indicate that there’s a gravitational lensing effect from the smaller
concentrations, in addition to that of the galaxy and its dark matter shell. In
this case, there are spatial fluctuations in the density of dark matter down to
a size of about 30,000 light-years.
-
- Such smaller
concentrations work with predictions made about cold dark matter (CDM). It says that dark matter clumps exist within
galaxies, but also can populate intergalactic space. The gravitational lensing
effects due to the clumps of dark matter found in this study are so small that
it is extremely difficult to detect them alone.
Astronomers used ALMA to detect the fluctuations. It can provide very
high-resolution radio observations of fluctuations caused by the smaller
concentrations of dark matter.
-
- Astronomers used a
giant radio telescope, the Five-Hundred-Aperture Spherical Radio Telescope
(FAST) in China to look at another interesting dark matter-related object that
lies near the galaxy M94. The system, which they call “Cloud-9, is a source of
21-centimeter radio emissions from cold neutral interstellar hydrogen.
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- Cloud-9 appears to
be relatively starless. Astronomers using FAST wondered if this 21-cm emission
from the cloud could function as a tracer of dark matter. Cloud-9 is very similar to a
“REionization-limited HI Cloud” (RELHIC). Essentially, a RELHIC is a starless
dark matter halo. It’s filled with gas in equilibrium with the cosmic
ultraviolet background. The “wash” of
UV radiation produced by stars and galaxies.
-
- The most popular
model of cold dark matter, galaxies form in the centers of halos of a certain
size. The models also show that there should be a number of collapsed dark
matter halos. And, there’s no reason why there shouldn’t be smaller “blobs”
throughout the Universe
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- Many low-mass
versions of dark matter halos remain dark or starless even after billions of
years of cosmic evolution. So, not every such halo contains a galaxy. The
RELHICs are halos that have no stars.
-
- Observing
Cloud-9 has measurements of the column
densities of gas there. Its characteristics point to it being associated with a
dark matter halo. If it is a cold dark matter RELHIC and it lies at (or near)
the distance of M94 (around 18 million light-years), that makes it one of the
closest RELHICs known.
-
- Future
observations using the MeerKAT telescope, the Very Large Array, or even the
FAST in China could be made. Follow-up observations using the Hubble Space
Telescope could help astronomers find out whether or not Cloud-9 definitely has
any stellar components.
-
- This mysterious
cloud could have a galaxy at its heart, but that it’s too faint to be detected.
Certainly in the early Universe, galaxies and stars formed as a result of
gravitational growth of density fluctuations in dark matter. Hydrogen and
helium (the building blocks of stars) were attracted by the clumps of dark
matter and began to form stars.
-
- However, that
applies to the distribution of dark matter at the galaxy and larger scales.
Accumulations of dark matter on smaller scales, such as with Cloud-9, really
aren’t well understood. That’s why further observations of Cloud-9 will
definitely help determine the full extent of its dark matter content. They
could also shed some light on how galaxies form at the small scale of smaller
dark matter accumulations.
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-
January 16, 2023
DARK MATTER
- new discoveries? 4319
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