New Theory About the Nature of Dark Matter Explains Mysterious Deficiency in Galaxy Pair

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New Theory About the Nature of Dark Matter Explains Mysterious Deficiency in Galaxy Pair

 

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

 

A new theory about the nature of dark matter helps explain why a pair of galaxies about 65 million light-years from Earth contains very little of the

 

mysterious matter, according to a study led by a physicist at the University of California, Riverside.

 

Dark matter is nonluminous and cannot be seen directly. Thought 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, making it difficult to detect.

 

The prevailing dark matter theory, known as cold dark matter, or CDM, assumes dark matter particles are collisionless, aside from gravity. A newer

 

second theory, called self-interacting dark matter, or SIDM, proposes dark matter particles self-interact through a new dark force. Both theories explain

 

how the overall structure of the universe emerges, but they predict different dark matter distributions in the inner regions of a galaxy. SIDM suggests

 

dark matter particles strongly collide with one another in a galaxy’s inner halo, close to its center.

 

Typically, a visible galaxy is hosted by an invisible dark matter halo — a concentrated clump of material, shaped like a ball, that surrounds the galaxy

 

and is held together by gravitational forces. Recent observations of two ultra-diffuse galaxies, NGC 1052-DF2 and NGC 1052-DF4, show, however, that

 

this pair of galaxies contains very little, if any, dark matter, challenging physicists’ understanding of galaxy formation. Astrophysical observations

 

suggest NGC 1052-DF2 and NGC 1052-DF4 are likely satellite galaxies of NGC1052.

 

 

Hai-Bo Yu is a theoretical physicist with expertise in the particle properties of dark matter. Credit: Samantha Tieu

 

“It is commonly thought that dark matter dominates the overall mass in a galaxy,” said Hai-Bo Yu, an associate professor of physics and astronomy at

 

UCR, who led the study. “Observations of NGC 1052-DF2 and -DF4 show, however, 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 considered that the DF2 and DF4 halos may be losing the majority of their mass

 

through tidal interactions with the massive NGC 1052 galaxy.” 

 

Using sophisticated simulations, the UCR-led team reproduced the properties of NGC 1052-DF2 and NGC 1052-DF4 through tidal stripping — the

 

stripping away of material by galactic tidal forces — by NGC1052. Because the satellite galaxies cannot hold the stripped mass with their own

 

gravitational forces, it effectively gets added to NGC 1052’s mass.

 

The researchers considered both CDM and SIDM scenarios. Their results, published in Physical Review Letters, indicate SIDM forms dark-matter-

 

deficient galaxies like NGC 1052-DF2 and -DF4 far more favorably than CDM, as the tidal mass loss of the inner halo is more significant and the stellar

 

distribution is more diffuse in SIDM. 

 

The research paper has been selected as an “editors’ suggestion” by the journal, an honor that only a select few papers receive each week to promote

 

reading across fields.

 

Yu explained tidal mass loss could occur in both CDM and SIDM halos. In CDM, the inner halo structure is “stiff” and resilient to tidal stripping, which

 

makes it difficult for a typical CDM halo to lose sufficient inner mass in the tidal field to accommodate observations of NGC 1052-DF2 and -DF4.  In

 

contrast, in SIDM, dark matter self-interactions could push dark matter particles from the inner to the outer regions, making the inner halo “fluffier” and

 

enhancing the tidal mass loss accordingly. Further, the stellar distribution becomes more diffuse.

 

“A typical CDM halo remains too massive in the inner regions even after tidal evolution,” Yu said.

 

Next, the team will perform a more comprehensive study of the NGC 1052 system and explore newly discovered galaxies with novel properties in an

 

effort to better understand the nature of dark matter.

 

Reference: “Self-Interacting Dark Matter and the Origin of Ultradiffuse Galaxies NGC1052-DF2 and -DF4” by Daneng Yang, Hai-Bo Yu and Haipeng An, 9

 

 

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