Dark Matter May Have Punched Huge Holes In A Stream Of Stars

Artistic impression of a galaxy surrounded by dark matter sub-halos. V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge). Photo: Color image of M31 from Adam Evans. Dark matter clumps from Aquarius, Volker Springel (HITS)

Researchers from the University of Cambridge have discovered two massive gaps in a stellar stream surrounding the Milky Way. The scientists believe that these holes were formed by clumps of dark matter (sub-halos) passing through the stream. If confirmed, these would be the smallest dark matter clumps ever.

"While we do not yet understand what dark matter is formed of, we know that it is everywhere," said lead author Dr Denis Erkal, from Cambridge's Institute of Astronomy, in a statement. "It permeates the universe and acts as scaffolding around which astrophysical objects made of ordinary matter – such as galaxies – are assembled."

In a paper, available online and submitted to the Monthly Notices of the Royal Astronomical Society, the astronomers have looked at the globular cluster Palomar 5, located over 60,000 light-years from the center of the Milky Way. This cluster is famous for a long stream of stars stretching out due to the Milky Way’s gravity.

"Stellar streams are actually simple and fragile structures," said co-author Dr Sergey Koposov in the statement. "The stars in a stellar stream closely follow one another since their orbits all started from the same place. But they don't actually feel each other's presence, and so the apparent coherence of the stream can be fractured if a massive body passes nearby.

"If a dark matter sub-halo passes through a stellar stream, the result will be a gap in the stream which is proportional to the mass of the body that created it."

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Comparison between the observed stream and simulations. V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge)

The team looked at this stellar stream hoping to see the influence of dark matter interactions and they found a pair of wrinkly stellar streams with two gaps. The team used simulations to estimate the size of these dark matter clumps and found that they could have been between one and 100 million solar masses.

"If dark matter can exist in clumps smaller than the smallest dwarf galaxy, then it also tells us something about the nature of the particles which dark matter is made of – namely that it must be made of very massive particles," said co-author Dr Vasily Belokurov. "This would be a breakthrough in our understanding of dark matter."

The true nature of dark matter remains elusive, but indirect evidence such as this could provide fundamental clues to aid our quest in our understanding of the dark side of the universe.

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