A galaxy floating next to our own some 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.
This mysterious substance accounts for more than 80% of the mass of the universe, but has not yet been directly observed.
Scientists say that the satellite universecalled Crater II, may consist of self-interacting dark matter (SIDM), a hypothetical variant of dark matter the particles are predicted to interact through some as yet unknown force outside gravity. This 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 how well it would work in explaining the Crater II observations,” study co-author Hai-Bo Yu, professor of physics and astronomy . at the University of California, Riverside, Space.com told us.
“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 images taken by the Very large telescope in Chile, the Crater II system is the fourth largest satellite in 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 scientist. Crater 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 several 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 is evolving over eons under the pull of the Milky Way’s gravity; our galaxy exerts a tidal force on the galaxy, stretching out its profile. These pulling forces also affect the dark matter halo — a spherical, invisible structure around Crater II — and the galaxy’s stars.
“A useful analogy is that the tidal force of the moon leads to ocean tides on the SoilYu said. ‘For the Milky Way’s satellites, the tidal force can pull away stars and dark matter, reducing the mass of the satellites over time.’
However, recent measurements of the galaxy’s orbit around the Milky Way suggest that those interactions are too weak to explain Crater II’s dark matter densities — that is, if dark matter consists of “cold,” collisionless particles, as predicted by the prevailing Lambda-CDM (LCDM or CDM) model of cosmology. Long-term tidal interactions with the Milky Way should also have caused Crater II to shrink more than observed, scientists say.
Using 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 forces of the Milky Way. 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 dense dark matter “knob” toward its center, as predicted by the LCDM model. On the other hand, if dark matter is indeed made of particles that interact with each other, collisions in the inner regions of a dark matter halo could 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 knob, the team leaders said. studywhich was published this month in The Astrophysical Journal Letters.
According to the researchers, SIDM also predicts that a galaxy will expand within the dark matter halo, which would explain the large size of Crater II better than the CDM models.
“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.”