Dark matter remains mysterious and…well…dark. Although we still don’t have a clear idea of what this “matter” in the universe is made of, astronomers are learning more about its distribution throughout the universe. Since we cannot see it directly, observers must use indirect methods to detect it. One method is through gravitational lensing. The other is by looking for emissions from hydrogen gas associated with small-scale dark matter structures in the universe.
A group of astronomers led by Kaikitaro Inoue of Kindai University in Japan used Chile’s Atacama Large Millimeter Array to study a distant gravitational lensing system called MG J0414+534. A giant galaxy in the foreground bends and distorts light from a distant quasar some 11 billion light-years away. The result is his four images of quasars. When examining the data, the research team discovered a strange anomaly in the images. These are actually changes in the distribution of dark matter along the line of sight between us and the quasar. Gravitational lenses magnified the fluctuations, and analysis of the data allowed him to map the fluctuations down to a scale of 30,000 light years.
What dark matter clumps mean
Throughout the universe, dark matter is associated with giant galaxies and galaxy clusters. However, small-scale clusters and distributions are poorly understood. Astronomers therefore want to find a way to map that smaller concentration. Gravitational lenses offer him one way to do that. In the case of MG J0414+0534, the position and shape of the lensed quasar image looks a little strange. These do not fit the model of gravitational lensing predicted when fitting numerical values for galaxies and their associated dark matter components.
This variation indicates that in addition to the gravitational lensing of the galaxy and its dark matter shell, there is also gravitational lensing due to smaller concentrations. In this case, there is spatial variation in the density of dark matter up to a size of about 30,000 light-years. This is much smaller than what scientists call the “cosmic scale” of larger concentrations (tens of billions of light years).
Interestingly, the team’s work shows that such small concentrations can be useful for predictions about cold dark matter (CDM). Basically, clumps of dark matter exist within galaxies, but they may also exist in intergalactic space. The gravitational lensing effect caused by the clumps of dark matter found in this study is extremely small, making it extremely difficult to detect on its own. That’s why the research team used ALMA to detect the fluctuations. It can provide very high-resolution radio observations of fluctuations caused by smaller concentrations of dark matter.
A starless dark matter halo in the local universe?
Astronomers used the 500 Aperture Spherical Radio Telescope (FAST), a giant radio telescope in China, to observe another interesting dark matter-related object near galaxy M94. The system, which they call “Cloud 9,” is a 21-centimeter source of radio radiation from cold, neutral interstellar hydrogen. Interestingly, Cloud-9 seems to have relatively few stars. So astronomers using FAST wondered if this 21 cm of radiation from the cloud could act as a tracker for dark matter. They explain in a preprint paper that Cloud-9 is very similar to what they call his REionization-limited HI Cloud (RELHIC). Essentially, RELHIC is a starless halo of dark matter. It is filled with gas that is in equilibrium with the cosmic ultraviolet background. (This is the “cleaning” of ultraviolet light produced by stars and galaxies.)
The most common model of cold dark matter shows that galaxies form at the center of a halo of constant size. The model also shows that there should be many halos of collapsed dark matter. Interestingly, even after billions of years of evolution of the universe, many of the lower mass versions remain dark or starless. So basically, not all such halos contain galaxies. RELHIC is a halo with no stars (or at least no stars detected so far).
Observe Cloud-9
A team led by astronomer Alejandro Benitez Lambray from the University of Milano Bococca in Italy reports that Cloud-9 is an extended HI cloud. They measured the column density of gas there. Its features indicate that it is associated with a dark matter halo. If it is a cold dark matter RELHIC and is located at (or near) the distance of M94 (about 18 million light-years), it would be one of the closest known RELHICs.
Cloud-9 needs more research, and the authors of the current study suggest that higher-resolution observations are needed. First, you need to figure out the distance really accurately. Therefore, in the future, observations may be made using the MeerKAT telescope, the Very Large Array, and even China’s FAST. Follow-up observations using the Hubble Space Telescope could help astronomers determine whether Cloud-9 definitely has a stellar component.
The authors suggest that there may be a galaxy at the heart of this mysterious cloud, but it may be too small to detect. Indeed, in the early Universe, galaxies and stars formed as a result of the gravitational growth of dark matter density fluctuations. Hydrogen and helium (the building blocks of stars) were attracted to the clump of dark matter and began to form stars. But it also applies to galaxies and the distribution of dark matter on larger scales. Small-scale dark matter accumulations like Cloud-9 are not really well understood. That’s why further observations of Cloud-9 will undoubtedly help reveal the full extent of its dark matter content. It may also shed light on how galaxies form in small-scale regions where smaller amounts of dark matter accumulate.
For more information
New cosmological constraints on the nature of dark matter
Measurement of 10 kpc scale lens power spectrum of lens quasar MG J0414+0534 using ALMA
Is the recently discovered HI cloud near M94 a starless dark matter halo?