Darkish Matter Slows Down the Rotation of the Milky Means’s Galactic Bar – Watts Up With That?
From University College London
According to a new study by researchers at UCL and the University of Oxford, the rotation of the Milky Way’s galactic bar, which is made up of billions of star clusters, has slowed by about a quarter since it was formed.
Artistic conception of the Milky Way. Source: Wikimedia Commons. Photo credit: Pablo Carlos Budassi.
Astrophysicists have been predicting such a slowdown for 30 years, but this is the first time it has been measured.
The researchers say it gives new insight into the nature of dark matter, which acts like a counterweight that slows rotation.
In the study, published in the Monthly Notices of the Royal Astronomical Society, the researchers analyzed observations from the Gaia Space Telescope of a large group of stars, the Hercules Current, that are in resonance with the rod – that is, they orbit the galaxy at the same speed as the spin of the bar.
These stars are gravitationally captured by the rotating rod. The same phenomenon occurs with Jupiter’s Trojan and Greek asteroids orbiting Jupiter’s Lagrangian points (in front of and behind Jupiter). If the rotation of the bar slowed, one would expect these stars to continue moving in the galaxy and their orbital time to match that of the rotation of the bar.
The researchers found that the stars in the stream bear a chemical fingerprint – they are richer in heavier elements (called metals in astronomy), proving that they have moved away from the galactic center, where stars and star-forming gas are about ten times as large how rich in metals compared to the outer galaxy.
From this data, the team concluded that the bar – made up of billions of stars and trillions of solar masses – had slowed its rotation by at least 24% since it was formed.
Co-author Dr. Ralph Schoenrich (UCL Mullard Space Science Laboratory) said, “Astrophysicists have long suspected that the rotating bar at the center of our galaxy is slowing down, but we have found the first evidence of this.
“The counterweight that slows this rotation must be dark matter. So far we could only infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from the visible matter.
“Our research offers a new way of measuring dark matter – not its gravitational energy, but its inertial mass (the dynamic response), which slows the rotation of the beam.”
Co-author and PhD student Rimpei Chiba of Oxford University said, “Our results offer a fascinating perspective to constrain the nature of dark matter as different models will alter this inertial force on the galactic bar.
“Our finding also poses a major problem for alternative theories of gravity – since they lack dark matter in the halo, they predict no or significantly too little slowing down of the bar.”
It is believed that the Milky Way, like other galaxies, is embedded in a “halo” of dark matter that extends well beyond its visible edge.
Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies that behave as if they were enveloped in significantly more mass than we can see. It is believed that there is about five times as much dark matter in the universe as ordinary, visible matter.
Alternative theories of gravity such as modified Newtonian dynamics reject the idea of dark matter and instead try to explain discrepancies through an optimization of Einstein’s general theory of relativity.
The Milky Way is a barred spiral galaxy with a thick star bar in the center and spiral arms that extend through the disk outside the bar. The bar rotates in the same direction as the galaxy.
The research was supported by the Royal Society, the Takenaka Scholarship Foundation, and the DiRAC Supercomputing Facility of the Science and Technology Facilities Council (STFC).
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