- 4318 - DARK MATTER - remains mysterious? 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.
------------------ 4318 - DARK MATTER - remains mysterious?
- 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.
-
- 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. The result is four images of the quasar.
-
- When astronomers
looked at the data, they found some strange anomalies in the images. They 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.
-
- 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 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.
-
- 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. 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. That’s why the team 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,
called “Cloud-9, is a source of 21-centimeter radio emissions from cold
neutral interstellar hydrogen.
-
- Cloud-9 appears to
be starless. Astronomers using FAST
wondered if this 21-cm emission from the cloud could function as a tracer of
dark matter. “REionization-limited HI
Cloud” (RELHIC) is a starless dark matter halo. It’s filled with gas in
equilibrium with the cosmic ultraviolet background. That’s 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.
-
- Many low-mass
versions of dark matter halos remain dark or starless even after billions of
years of cosmic evolution. Not every
such halo contains a galaxy. The RELHICs are halos that have no stars, or at
least none that have been detected so far.
-
- Astronomers
measured the column densities of gas. 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.
-
-
January 17, 2023
DARK MATTER
- remains mysterious? 4318
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