Ancient Dark-Matter Storm –“Heading Towards Our Sun” (A 2018 Most Viewed)

Dark Matter Clumps


This past fall, a team of astronomers detected 10 billion solar masses worth of dark matter from an ancient dwarf galaxy, Gaia-Enceladus, swallowed by the Milky Way billions of years ago carried along by the S1 star stream that harbors our Solar System streaming directly towards the Sun.

This strange dark matter phenomena is speeding towards the Sun at speeds of 500 kilometers per second according to a study published in the journal Physical Review D, led by theoretical physicist Ciaran O’Hare from the University of Zaragoza in Spain.

A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster, or in this case the S1 Stream of an ancient dwarf galaxy, that has been ripped apart and stretched out along its orbit by tidal forces. “(There are) tons of these streams all over the galaxy, some of them are really huge and you can see them in the sky,” said O’Hare.

S1 Star Stream

The illustration above shows a small galaxy being torn apart as it is consumed by a larger galaxy, like the Milky Way. Gradually, the stars and dark matter that belong to the smaller galaxy are mixed into the Milky Way halo. Our galaxy is embedded in a cloud of dark matter, thought to consist of tiny particles traveling along orbits through the halo, permeating all regions of the galaxy, extending far beyond the edge of the bright central spiral, but also orbiting through our solar system.[Jon Lomberg in collaboration with David Martinez-Delgado for the Stellar Tidal Stream Survey]

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The European Space Agency’s billion-star survey using the Gaia spacecraft zoomed in on the S1 stream because its some 30,000 stars have a different chemical composition than those native to our galaxy. While there are over 30 such streams known in our galaxy, S1 captured the intense interest of astronomers because our solar system is actually inside this stream. Similar elliptical paths will intersect for millions of more years.

Gaia revealed the vast stream of stars, the remnant of the ancient Gaia-Enceladus dwarf galaxy. traveling along similar orbits around the center of the Milky Way : highly elongated, tipped to the plane of the galaxy’s disk, and weirdly in a direction backward relative to other stars. This structure is so big that we’re actually inside it — literally, it surrounds the Sun in all directions — and can be seen stretching nearly across the entire sky.

“What we want to do is add the stream as part of our kind of main prediction for the types of signal that should show up in a dark matter experiment,” O’Hare said. According to a statement, current detectors searching for weakly interacting massive particles (WIMPs) (one popular idea of what dark matter might be) probably won’t see anything from S1, but future tech might.

O’Hare and colleagues looked at data captured by the liquified xenon detector, the LZ experiment located at the Sanford Underground Research Facility in South Dakota, US – and found the stream could be detected above the standard wind if it made up 10% of local dark matter, and the particles were between five and 25 times the mass of a proton.

The counter-rotating structure of the S1 stream will dramatically increase the amount of dark matter appearing to come from the same patch of sky as the standard dark matter wind, producing a tell-tale ‘ring’ like structure around this wind, something that directional dark matter detectors such as the multinational CYGNUS collaboration could easily detect in future.

The image at the top of the page is an artist’s impression of dark matter clumps around a Milky Way-like galaxy. 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)

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