The so-called self-interacting dark matter theory helps explain why NGC 1052-DF2 and NGC 1052-DF4, a pair of ultra-diffuse galaxies located approximately 65 million light-years away the constellation of Cetus, contain little dark matter.
The prevailing dark matter theory, known as cold dark matter, or CDM, assumes dark matter particles are collisionless, aside from gravity.
A newer 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. Recent observations of NGC 1052-DF2 and NGC 1052-DF4 (DF2 and DF4 for short) show, however, that these ultra-diffuse galaxies contain very little, if any, dark matter.
“It is commonly thought that dark matter dominates the overall mass in a galaxy,” said Dr. Hai-Bo Yu, a researcher in the Department of Physics and Astronomy at the University of California, Riverside.
“Observations of 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 galaxy NGC 1052.”
Using sophisticated simulations, Dr. Yu and colleagues from Tsinghua University reproduced the properties of DF2 and DF4 through tidal stripping — the stripping away of material by galactic tidal forces — by NGC 1052.
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 and found that the latter scenario forms dark-matter-deficient galaxies 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.
“Tidal mass loss could occur in both CDM and SIDM halos,” Dr. Yu said.
“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 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.”
The team’s work was published in the journal Physical Review Letters.
Daneng Yang et al. 2020. Self-Interacting Dark Matter and the Origin of Ultradiffuse Galaxies NGC1052-DF2 and -DF4. Phys. Rev. Lett 125 (11): 111105; doi: 10.1103/PhysRevLett.125.111105
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