Dark Matter, the mysterious invisible mass theorized to account for 85% of matter in the Universe, continues to elude scientists. While there is plenty of indirect evidence for its existence – the rotational curves of galaxies, galactic halos, and gravitational lenses – direct detection and the identification of its constituent particles remain elusive. Nevertheless, “Cold” Dark Matter is an integral part of the Standard Model of Cosmology, known as the Lambda Cold Dark Matter (ΛCDM) model.
In a recent study led by UC Riverside professor Hai-Bo Yu, a new type of dark matter is proposed that can explain three astrophysical mysteries in vastly different fields. In essence, the study proposed that dense clumps of Self-Interacting Dark Matter (SIDM) can account for the gravitational effects of gravitational lenses, stellar streams, and satellite galaxies. The team’s study, “Core-Collapsed SIDM Halos as the Common Origin of Dense Perturbers in Lenses, Streams, and Satellites,” was published in *Physical Review Letters*
Whereas CDM is “collisionless,” meaning that its particles pass through one another without interacting, SIDM is composed of particles that collide and exchange energy.
These interactions, according to Yu, who is also the deputy director of the Center for Experimental Cosmology and Instrumentation (CECI), can lead to “gravothermal collapse,” where particles form extremely dense, compact cores a million times the mass of the Sun. As he explained:
The difference is like a crowd of people who ignore each other versus one where everyone is constantly bumping into one another. In SIDM, these interactions can dramatically reshape the internal structure of dark matter halos. Dark matter that interacts with itself can become dense enough to explain these observations.
*The gravitational lens system JVAS B1938+666, based on data from Keck/EVN/GBT/VLBA. Credit: Devon Powell/MPA*
As Yu demonstrates, the SIDM model can simultaneously explain three observational phenomena. First, there’s the ultra-dense object in JVAS B1938+666, a well-known gravitational lens system consisting of a foreground galaxy 6.5 to 10 billion light-years from Earth, and a distant background galaxy that appears as an Einstein Ring. Second, there’s GD-1, a moving group of old and metal-poor stars (aka. “stellar stream”) that has several gaps and a ‘spur’ feature where part of the stream branches off from the main body.
Both of these features suggest the stream was perturbed by another object, potentially clumps of SIDM. Last, there’s the Fornax 6 globular star cluster in the Fornax dwarf galaxy, a satellite of the Milky Way. Unlike the other clusters that make up this galaxy, Fornax 6 is more metal-rich and therefore likely to be younger (ca. 2 billion years old). What’s more, six globular clusters are an unusually high number for a dwarf galaxy the size of Fornax. A dense clump of SIDM could explain how passing stars have been swept into tight clusters.
“What’s striking is that the same mechanism works in three completely different settings — across the distant universe, within our galaxy, and in a neighboring satellite galaxy,” Yu concludes. “All show densities that are difficult to reconcile with standard model dark matter but arise naturally in SIDM.” The research was supported by the John Templeton Foundation and the U.S. Department of Energy (DoE).
Further Reading: UC News