Physicists explain a stellar stream’s distinctive features

(Press-News.org) RIVERSIDE, Calif. -- Physicists have proposed a solution to a long-standing puzzle surrounding the GD-1 stellar stream, one of the most well-studied streams within the galactic halo of the Milky Way, known for its long, thin structure, and unusual spur and gap features. 

The team of researchers, led by Hai-Bo Yu at the University of California, Riverside, proposed that a core-collapsing self-interacting dark matter (SIDM) “subhalo” — a smaller, satellite halo within the galactic halo — is responsible for the peculiar spur and gap features observed in the GD-1 stellar stream. 

Study results appear in The Astrophysical Journal Letters. The research could have significant implications for understanding the properties of dark matter in the universe.

A stellar stream is a group of stars moving collectively along a shared trajectory. A gap refers to a localized under-density of stars along the stream, while a spur is an over-density of stars extending outward from the main body of the stream. Since dark matter governs the motion of stellar streams, astronomers can use them to trace invisible dark matter in a galaxy.

The Milky Way’s galactic halo, a roughly spherical region surrounding the galaxy, contains dark matter and extends beyond the galaxy’s visible edge. Visualizations of well-known stellar streams in the Milky Way can be found here.

Astronomers discovered that the spur and gap features of the GD-1 stellar stream cannot be easily attributed to the gravitational influence of known globular clusters or satellite galaxies of the Milky Way. These features could be explained, however, by an unknown perturbing object, such as a subhalo. But the object’s density would need to be significantly higher than what is predicted by traditional cold dark matter (CDM) subhalos.

“CDM subhalos typically lack the density needed to produce the distinctive features observed in the GD-1 stream,” explained Yu, a professor of physics and astronomy. “However, our research demonstrates that a collapsing SIDM subhalo could achieve the necessary density. Such a compact subhalo would be dense enough to exert the gravitational influence required to account for the observed perturbations in the GD-1 stream.”

Dark matter, which cannot be seen directly, is thought to make up 85% of matter in the universe. Its nature is not well understood. CDM, the prevailing dark matter theory, assumes dark matter particles are collisionless. SIDM, a theoretical form of dark matter, proposes dark matter particles self-interact through a new dark force. 

In their study, Yu and his team used numerical simulations called N-body simulations to model the behavior of a collapsing SIDM subhalo. 

“Our team’s findings offer a new explanation for the observed spur and gap features in GD-1, which have long been thought to indicate a close encounter with a dense object,” Yu said. “In our scenario, the perturber is the SIDM subhalo, which disrupts the spatial and velocity distributions of the stars in the stream and creates the distinctive features we see in the GD-1 stellar stream.”

According to Yu, the discovery also provides insights into the nature of dark matter itself. 

“This work opens a promising new avenue for investigating the self-interacting properties of dark matter through stellar streams,” he said. “It marks an exciting step forward in our understanding of dark matter and the dynamics of the Milky Way.”

Yu was joined in the research by Xingyu Zhang and Daneng Yang at UCR; and Ethan O. Nadler at the University of California, San Diego.

The work was supported by the U.S. Department of Energy and the John Templeton Foundation.  

The title of the research paper is “The GD-1 Stellar Stream Perturber as a Core-collapsed Self-interacting Dark Matter Halo.”

The University of California, Riverside is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment is more than 26,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual impact of more than $2.7 billion on the U.S. economy. To learn more, visit www.ucr.edu.

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