A core-collapsing self-interacting dark matter subhalo is responsible for the peculiar spur and gap features observed in GD-1, a stellar stream within the Galactic halo of the Milky Way, according to a new study led by University of California, Riverside astronomers.
GD-1 exhibits spur and gap structures that may result from a close encounter with a dense substructure. Image credit: University of California, Riverside.
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 halo, a roughly spherical region surrounding our Galaxy, contains dark matter and extends beyond the Galaxy’s visible edge.
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,” said University of California, Riverside’s Professor Hai-Bo Yu.
“However, our research demonstrates that a collapsing self-interacting dark matter (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.”
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, Professor Yu and his colleagues 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,” Professor 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.”
This 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,” Professor Yu said.
“It marks an exciting step forward in our understanding of dark matter and the dynamics of the Milky Way.”
The study appears in the Astrophysical Journal Letters.
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Xingyu Zhang et al. 2025. The GD-1 Stellar Stream Perturber as a Core-collapsed Self-interacting Dark Matter Halo. ApJL 978, L23; doi: 10.3847/2041-8213/ada02b