Two-particle dark matter model offers one answer to galaxy puzzles

A new model from researchers at Purple Mountain Observatory proposes that dark matter may be made of at least two kinds of particles, rather than one. Science China Press said the idea could connect two long-running astronomy problems: faintly concentrated dark matter in some dwarf galaxies and dense clumps inferred from gravitational lensing.

The study, published in Science Bulletin, was led by Daneng Yang, Yi-Zhong Fan, Siyuan Hou and Yue-Lin Sming Tsai. It builds on earlier work by the same team in Physical Review D on how this kind of dark matter behavior could affect the range of core densities seen in dwarf galaxies.

A split in the dark sector

Dark matter has not been observed directly, but astronomers infer its presence from its gravitational effects on galaxies and on the large-scale structure of the universe. For decades, the main framework has been the cold dark matter model, which has been successful in explaining broad features of cosmic structure, according to Science China Press.

More detailed observations have also highlighted tensions for that standard picture. Science China Press pointed to two examples: some dwarf galaxies appear to have lower dark matter densities at their centers than expected, while strong gravitational lensing points to compact dark matter structures on small scales.

The Purple Mountain Observatory team’s proposed model is called two-component self-interacting dark matter. In it, dark matter includes a heavier particle type and a lighter particle type, and the particles can interact with each other directly in addition to exerting gravity.

Those interactions produce what the researchers call mass segregation. In the model, heavier dark matter particles tend to move inward toward galactic centers over time, while lighter particles are pushed farther out. Science China Press compared the behavior to star clusters, where heavier stars can sink inward as lighter stars move outward.

Simulations link cores and lenses

Using high-resolution computer simulations and theoretical modeling, the team found that mass segregation could reproduce several observed features, according to Science China Press. In dwarf galaxies, the process can generate broad dark matter cores with relatively low central densities.

In other settings, the same mechanism can make some dark matter halos more compact. Science China Press said those denser structures would be better able to act as small-scale gravitational lenses, bending and magnifying light from more distant galaxies.

The researchers said this could raise the predicted number of small-scale strong lensing events. That matters because astronomers have observed more such lensing signals than traditional models predict, according to Science China Press.

The proposal does not directly detect dark matter particles or settle what they are. It offers a testable explanation for why dark matter may appear diffuse in some places and concentrated in others, using one set of particle properties.

More surveys could test the idea

Science China Press said future sky surveys and lensing observations may provide sharper tests of the model. Strong gravitational lensing, in particular, could help reveal whether small dark matter structures behave as expected under a multi-component, self-interacting model.

Purple Mountain Observatory is part of the Chinese Academy of Sciences and is active in dark matter research, including indirect detection work connected to the DAMPE, or Wukong, satellite. The new paper is titled “Self-interacting dark matter with mass segregation: a unified explanation of dwarf cores and small-scale lenses.”

This story draws on original reporting from ScienceDaily.