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A new theory about the nature of dark matter explains the mysterious deficit in the galactic pair



Concept for astrophysics of dark matter

A study led by UC Riverside found that the theory of self-interacting dark matter explains why two galaxies have less dark matter than others.

A new theory of the nature of dark matter helps explain why a pair of galaxies about 65 million light-years from Earth contain very little of the mysterious matter, according to a study led by a physicist at the University of California, Riverside.

Dark matter is opaque and cannot be seen directly. Considered to make up 85% of matter in the universe, its nature is not well understood. Unlike normal matter, it does not absorb, reflect or emit light, which makes it difficult to detect.

The prevailing theory of dark matter, known as cold dark matter or CDM, assumes that dark matter particles are colliding except for gravity. A newer second theory, called self-interacting dark matter, or SIDM, suggests that dark matter particles interact through a new dark force. Both theories explain how the overall structure of the universe came into being, but they predict different distributions of dark matter in the inner regions of the galaxy. SIDM suggests that particles of dark matter collide strongly with each other in the galaxy̵

7;s inner halo, near its center.

Hi-Bo Yu, University of California - Riverside

Hai-Bo Yu is a theoretical physicist with experience in the properties of dark matter particles. Credit: Samantha Tiu

Usually a visible galaxy hosts an invisible halo of dark matter – a concentrated clump of material shaped like a ball that surrounds the galaxy and is held together by gravitational forces. However, recent observations of two superdiffuse galaxies, NGC 1052-DF2 and NGC 1052-DF4, show that this pair of galaxies contains very little, if any, dark matter that causes physicists to understand galaxy formation. Astrophysical observations suggest that NGC 1052-DF2 and NGC 1052-DF4 are probable satellite galaxies of NGC1052.

“Dark matter is generally thought to dominate the total mass in a galaxy,” said Hi-Bo Yu, an associate professor of physics and astronomy at UCR who led the study. However, observations of NGC 1052-DF2 and -DF4 show that the ratio of their dark matter to their stellar masses is about 1, which is 300 times lower than expected. To resolve the discrepancy, we thought that the halos DF2 and DF4 could lose most of their mass through tidal interactions with the massive galaxy NGC 1052.

Using sophisticated simulations, the UCR-led team reproduced the properties of NGC 1052-DF2 and NGC 1052-DF4 by tidal stripping – removing material from galactic tidal forces – by NGC1052. Because satellite galaxies cannot hold the bare mass with their own gravitational forces, it is effectively added to the mass of NGC 1052.

The researchers looked at both CDM and SIDM scenarios. Their results, published in Physical Review Letters, show that SIDM forms dark matter-deficient galaxies such as NGC 1052-DF2 and -DF4 far more favorably than CDM, as the loss of tidal mass of the inner halo is more significant and the distribution of stars is more diffuse in SIDM.

The research article was selected as a “suggestion of the editors” by the journal, it is an honor that only a few selected articles receive each week to promote reading in various fields.

Yu explained that tidal mass loss can occur in both CDM and SIDM halos. In CDM, the internal structure of the halo is “rigid” and elastic to tidal bands, which makes it difficult for the typical CDM halo to lose enough internal mass in the tidal field to accommodate the observations of NGC 1052-DF2 and -DF4. In contrast, in SIDM, the interactions of dark matter can push the particles of dark matter from the inner to the outer areas, making the inner halo “fluffy” and thus increasing the loss of tidal mass. In addition, the stellar distribution becomes more diffuse.

“The typical CDM halo remains too massive in the interior, even after tidal evolution,” Yu said.

The team will then conduct a more comprehensive study of the NGC 1052 system and explore newly discovered galaxies with new properties in an attempt to better understand the nature of dark matter.

Reference: “Interacting Dark Matter and the Origin of the Ultradiffuse Galaxies NGC1052-DF2 and -DF4” by Daneng Yang, Hai-Bo Yu and Haipeng An, September 9, 2020, Physical examination letters.
DOI: 10.1103 / PhysRevLett.125.111105

Yu joined the study from Daneng Yang and Haipeng An from Tsinghua University in Beijing, China. Yu was supported by a grant from the US Department of Energy and the US National Science Foundation.




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