ALMA reveals a subtle signature of dark matter

Pioneering observations reveal dark matter fluctuations below the level of galaxies, confirming theories of cold dark matter and providing new insights into the formation of the universe.

A research team led by Professor Kaiki Taro Inoue at Kindai University (Osaka, Japan) has detected fluctuations in the distribution of dark matter in the universe on scales smaller than massive galaxies using the world’s most powerful radio interferometer, the Atacama Large Millimeter/submillimeter Array. Array).Alma), located in the Republic of Chile.

This is the first time that spatial fluctuations of dark matter have been detected in the distant universe on a scale of 30 thousand light-years. This result shows that cold dark matter^[1] This is preferable even on scales smaller than massive galaxies, and is an important step toward understanding the true nature of dark matter. The article will be published in the Astrophysical Journal.

Figure 1. Fluctuations detected in dark matter. Brighter orange color indicates areas with high dark matter density and darker orange color indicates areas with low dark matter density. The white and blue colors represent gravitationally lensed objects observed by ALMA. Source: ALMA (ESO/NAOJ/NRAO), KT Inoue et al.

the main points

• Observation by one of the world’s largest radio wave interferometers ALMA, an international project.
• The first detection of dark matter fluctuations in the universe on scales of less than 30 thousand light-years.
• An important step towards clarifying the true nature of dark matter.

ALMA detects small-scale fluctuations in the distribution of dark matter

Dark matter, the invisible matter that makes up much of the universe’s mass, is thought to have played an important role in the formation of structures such as stars and galaxies.^[2] Since dark matter is not uniformly distributed in space but distributed in clumps, its gravity can slightly alter the path of light (including radio waves) coming from distant light sources. Observations of this effect (gravitational lensing) have shown that dark matter is associated with relatively massive galaxies and galaxy clusters, but how it is distributed on smaller scales is not yet known.

The research team decided to use ALMA to observe an object 11 billion light-years from Earth. The object is a lensed quasar,^[3] MG J0414+0534^[4] (hereinafter referred to as “this quasar”).

This quasar appears to have a quadrilateral image due to the gravitational lensing effect of the foreground galaxy. However, the positions and shapes of these apparent images differ from those calculated solely from the gravitational lensing effect of the foreground galaxy, suggesting that the gravitational lensing effect of dark matter distribution on scales smaller than massive galaxies is at work.

Figure 2: Conceptual diagram of the MG J0414+0534 gravitational lens system. The object in the center of the image indicates a lenticular galaxy. Orange shows dark matter in intergalactic space, and pale yellow indicates dark matter in a lenticular galaxy. Credit: NAOJ, KT Inoue

It was found that there are spatial fluctuations in the density of dark matter even on a scale of about 30 thousand light-years, which is much less than the cosmic scale (several tens of billions of light-years). This result is consistent with the theoretical prediction of cold dark matter, which predicts that clumps of dark matter exist not only inside galaxies (pale yellow in Figure 2), but also in intergalactic space (orange in Figure 2).

The gravitational lensing effects of the dark matter clumps found in this study are so small that they are extremely difficult to detect on their own. However, thanks to the gravitational lensing effect caused by the foreground galaxy and the high resolution of ALMA, we were able to detect the effects for the first time. Therefore, this research is an important step to verify the theory of dark matter and clarify its true nature.

This research is presented in a paper titled “ALMA measurement of lensing power spectra at 10 kpc toward the lensed quasar MG J0414+0534” by KT Inoue et al. In the Astrophysical Journal.

Notes

1. Cold dark matter
   As the universe expands, the density of matter decreases, so dark matter particles (matter invisible to light) will no longer encounter other particles and will have an independent motion different from the motion of ordinary matter. In this case, dark matter particles that move at a speed much lower than the speed of light relative to ordinary matter are called cold dark matter. Because of their low speed, they do not have the ability to erase large-scale structures in the universe.
2. Formation of structure in the universe
   In the early universe, stars and galaxies are thought to have formed as a result of gravitational growth of dark matter density fluctuations, and the accumulation of hydrogen and helium being attracted to clumps of dark matter. The distribution of dark matter on scales smaller than massive galaxies is still unknown.
3. Quasar
   A quasar is the central, compact region of a galaxy that emits extremely bright light. The combined area and its surroundings contain a large amount of dust that emits radio waves.
4. MG J0414+0534
   MG J0414+0534 is located in the direction of the constellation Taurus as seen from Earth. The redshift (increase in the wavelength of light divided by the original wavelength) of this object is z=2.639. The corresponding distance is assumed to be 11 billion light-years, taking into account the uncertainty in cosmological parameters.

Reference: “ALMA measurement of lensing force spectra at 10 kpc toward lensed quasar MG J0414+0534” by Kaiki Taro Inoue, Takeo Minezaki, Satoki Matsushita, and Koichiro Nakanishi, 7 September 2023, Astrophysical Journal.
doi: 10.3847/1538-4357/aceb5f

This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 17H02868, 19K03937), the National Astronomical Observatory of Japan ALMA Joint Scientific Research Project 2018-07A, the same as the ALMA JAPAN Research Fund NAOJ-ALMA-256, and Taiwan MoST. 103-2112-M-001-032-MY3, 106-2112-M-001-011, 107-2119-M-001-020, 107-2119-M-001- 020.