A strange dark-matter phenomenon is speeding towards the Sun at speeds of 500 kilometers per second according to a 2018 study led by theoretical physicist Ciaran O’Hare from the University of Zaragoza in Spain. Billions of years ago, a dwarf galaxy was shred apart by the extreme tidal forces of our larger Milky Way Galaxy. The remnant galaxy now forms a stream, called S1, that arcs around the halo of our Galaxy. The stream is composed of tens of thousands of visible stars, and also up to a billion solar masses of invisible dark matter.
Intersects Precisely at Our Solar System
“(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. What makes stream S1 special — it intersects the plane of the Milky Way’s disk at precisely the location of our solar neighborhood.
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 (mergers and tidal interactions between massive galaxies and their dwarf satellites are a fundamental prediction of the Lambda-Cold Dark Matter cosmology).
Milky Way Embedded in a Cloud of Dark Matter
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]
One of 30 Star Streams in the Milky Way
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.
Detecting a Dark-Matter Signal
“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 be able to distinguish S1 dark matter from the normal “wind” of dark matter that crosses Earth as our solar system orbits the Milky Way. But future tech might.
O’Hare and colleagues looked at data captured by the liquid 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.
A Ring-Like Structure
As in the illustration above, the S1 stream is on a low-inclination orbit and in the opposite direction as the solar system’s path around the Milky Way. 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.