A galaxy floating next to our own, some 380,000 light-years from Earth, could offer new clues in the 90-year-long quest to determine the nature of dark matter, the invisible glue that holds galaxies together.
The mysterious substance makes up more than 80% of the mass of the universe, but has not yet been directly detected.
Scientists say that the satellite galaxynamed Crater II, may consist of self-interacting dark matter (SIDM), which is a hypothetical variety dark matter whose particles are predicted to interact through an as yet unknown force outside gravitation. This hypothesis has gained attention in recent years as an alternative form of conventional “cold” dark matter.
“When we started this project, we knew roughly how SIDM would work, but we had no idea how well it would work in explaining the Crater II observations,” study co-author Hai-Bo Yu, who is a professor of physics and astronomy. at the University of California, Riverside, told Space.com.
“Our computer simulations of Crater II analogs show that the agreement between [self-interacting dark matter] predictions and observations of Crater II is surprisingly good, and the required dark matter self-interaction strength is larger than we originally assumed.”
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Discovered in 2016 on the pictures taken A very large telescope in Chile, the Crater II galaxy is the fourth largest satellite Milky Way — after the Large and Small Magellanic Clouds and the Sagittarius Galaxy. If it were visible to the naked eye, he said, it would appear twice as large as the full moon The new scientist. Crater II hosts several billion old stars that are spread over 6,500 light-years, making the “faint giant” remarkably faint—almost 100,000 times fainter than the Milky Way.
Despite many attempts to simulate the properties of Crater II over the years, it remains unclear how the galaxy formed and how it has maintained its relatively large size. Astronomers know that Crater II has been developing over eons under the Milky Way’s gravitational influence; our galaxy exerts a tidal force on it that stretches its profile. These tugs also affect its dark matter halo—the spherical, invisible structure surrounding Crater II—as well as the galaxy’s stars.
“A useful analogy is tidal power Moon leads to ocean tides on Earth“For Milky Way satellites, the tidal force can pull stars and dark matter away, reducing the mass of the satellites over time.”
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However, recent measurements of the galaxy’s orbit around the Milky Way suggest that these interactions are too weak to explain the dark matter densities in Crater II—that is, if the dark matter is composed of “cold” collisionless particles, as predicted by the prevailing Lambda-CDM (LCDM or CDM) model of cosmology. Prolonged tidal interactions with the Milky Way should also shrink Crater II more than observed, the researchers say.
Based on measurements of Crater II’s orbit, Yu and other team members simulated the mass loss of stars and dark matter particles due to the tidal force of the Milky Way. The team found that the observed properties of the galaxy could be explained by dark matter particles interacting with each other.
Crucially, Crater II does not have a high-density dark matter “spike” at its center, as predicted by the LCDM model. On the other hand, if dark matter were indeed made of self-interacting particles, collisions in the inner regions of the dark matter halo can transfer energy between the particles “and tend to make them carry the same amount of energy,” Yu said. This would somehow offset the halo in Crater II and explain the lack of a central spike, according to the team studieswhich was published this month in The Astrophysical Journal Letters.
SIDM also predicts that the galaxy will expand in a dark matter halo, which would explain the large size of Crater II better than CDM models, the researchers say.
“Our work shows that SIDM can well explain the unusual features of Crater II that challenge CDM,” Yu said. “To further confirm whether dark matter really carries the new force, we hope to see more galaxies like Crater II.”