Our solar system is plunging through a galaxy cannibalised by the Milky Way, raising the prospects for detection of its dark matter remains.
The Milky Way, and indeed all galaxies, formed within a vast halo of invisible additional mass called dark matter, which outweighs the visible component five times over.
The motion of the solar system around the Milky Way means it moves through this dark matter halo at 230 kilometres per second. The dark matter, thus, appears to us as a high-speed “wind”.
Last year, the motions of nearby stars in the solar neighbourhood were measured by the European Space Agency’s Gaia satellite and a previously unknown stream, dubbed S1, was detected – the telltale remains of a smaller dwarf galaxy cannibalised by the Milky Way. Now a study published in the journal Physical Review D, led by Ciaran O’Hare from the University of Zaragoza in Spain, finds that 10 billion solar masses worth of dark matter from that galaxy is travelling along S1, directly towards the Sun.
This dark matter should strike the Sun – and any detectors on Earth – at speeds of 500 kilometres per second – much faster than the standard dark matter wind. O’Hare and colleagues call it a “dark matter hurricane”.
The study explores several popular candidates for the as yet unknown particle that makes up the dark matter to test how this hurricane would impact direct detection experiments.
The standard case posits a weakly interacting massive particle or WIMP, from a few to hundreds of times the mass of a proton, that collides with atoms to produce a visible nuclear recoil. These are currently the target for several sodium iodide crystal and liquified xenon detectors.
O’Hare and colleagues looked at one of the latter – 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.
As the S1 stream “hits the solar system slap in the face”, the authors write, its counter-rotating structure will dramatically increase the amount of dark matter appearing to come from the same patch of sky as the standard dark matter wind. Indeed, it should produce 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.
Finally, the most dramatic sign of the hurricane was found for the case of exotic dark matter particles known as axions. These superlight candidates can be converted into photons in the presence of intense magnetic fields. They are rapidly gaining favour among dark matter hunters worldwide.
Whatever the elusive particle turns out to be, the prospects of its discovery have improved since the discovery of S1. O’Hare and colleagues say the onrushing hurricane will increase dark matter detection prospects “substantially”.
Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. He was Lead Scientist of The Royal Institution of Australia from 2017 to 2021. Twitter | @astroduff
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