A galaxy orbiting next to our own about 380,000 light-years from Earth could provide new clues in the 90-year search for the nature of dark matter, the invisible glue that holds galaxies together.
The mysterious substance is responsible for more than 80% of the universe’s mass, but has yet to be directly detected.
Scientists say the satellite universecalled Crater II, may consist of self-interacting dark matter (SIDM), which is a hypothetical variant of dark matter the particles of which are predicted to interact with each other via an as yet unknown force gravityThis hypothesis has attracted 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 whether it would explain the observations of Crater II well,” said study co-author Hai-Bo Yu, professor of physics and astronomy at the University of California in Riverside, to Space.com.
“Our computer simulations of Crater II analogs show that the similarity between [self-interacting dark matter] “The predictions and observations of Crater II are surprisingly good, and the required strength of the dark matter interaction is larger than we initially expected.”
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Discovered in 2016 in photos taken by the Very large telescope In Chile, the Crater II constellation is the fourth largest satellite of the Milky Way — after the Large and Small Magellanic Clouds and the Sagittarius constellation. If it were visible to the naked eye, it would appear twice as large as the full moon, according to New scientistCrater II is home to several billion old stars spread across 6,500 light-years, making the “faint giant” remarkably dim – nearly 100,000 times fainter than the Milky Way.
Despite multiple attempts over the years to simulate the properties of Crater II, it remains unclear how the galaxy formed and maintains its relatively large size. Astronomers know that Crater II develops over eons under the influence of the Milky Way’s gravity; our galaxy exerts a tidal force on the galaxy, stretching its profile. These pulling forces also affect the dark matter halo — a spherical, invisible structure surrounding Crater II — and the galaxy’s stars.
“A useful analogy is that the tidal force of the moon leads to ocean tides on the Soil” said Yu. “For the Milky Way’s satellites, the tidal force can tear away stars and dark matter, causing the satellites’ mass to decrease 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 density in Crater II—that is, if dark matter is made up of “cold,” collisionless particles, as predicted by the prevailing Lambda Theory-CDM (LCDM or CDM) cosmology model. Long-term tidal interactions with the Milky Way also could have caused Crater II to shrink more than observed, scientists 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 Milky Way’s tidal force. The team found that the observed properties of the galaxy can be explained by dark matter particles interacting with each other.
Crucially, Crater II does not show a “knot” of high-density dark matter towards the center, as predicted by the LCDM model. On the other hand, if dark matter is indeed composed of interacting particles, collisions in the inner regions of a dark matter halo can transfer energy between the particles “and tend to make them carry the same amount of energy” , Yu said. That would smooth out Crater II’s halo somewhat and explain the lack of a central cusp, the team leaders said. studywhich was published this month in The Astrophysical Journal Letters.
SIDM also predicts that a galaxy will expand within the dark matter halo, which would explain Crater II’s large size better than CDM models, the researchers say.
“Our work shows that SIDM can well explain the unusual properties of Crater II that challenge CDM,” Yu said. “To further confirm whether dark matter indeed brings a new force, we hope to see more galaxies like Crater II.”