Astronomers Reveals That Dark Matter Is Slowing Down The Spin Of The Milky Way's Galactic Bar

The Milky Way rotates at a staggering 130 miles per second, but a new study discovered that dark matter has slowed the rotation of its bar by at least 24 percent since its formation nearly 14 billion years ago.

"Astrophysicists have long suspected that the spinning bar at the center of our galaxy is slowing down, but we have found the first evidence of this happening," study co-author Ralph Schoenrich, an astrophysicist at University College London, said in a statement.

These discoveries shed light on the Milky Way rotation and the nature of one of the universe's most elusive materials — dark matter.

The study, published in the Monthly Notices of the Royal Astronomical Society, examined Gaia space telescope observations of a large group of stars known as the Hercules stream that resonates with the bar, meaning they revolve around the galaxy at the same rate as the bar's spin.

Astronomers have long debated whether the galactic bar's rotation speed is increasing or decreasing. Rimpei Chiba of the University of Oxford and Ralph Schönrich of the University College London argues that it is slowing. They have calculated the rate of its gradual deceleration using observations of stars from the Gaia space telescope.

Many spiral galaxies, including the Milky Way, have a central bar-shaped region densely packed with stars and surrounded by the galaxy's pinwheeling arms. The bar also has some groupies: a crew of stars trapped by the bar's gravitational pull; these stars orbit a gravitationally stable point known as a Lagrange point, where the gravity from the galactic bar and the outward push from its rotation balance to create a stable gravitational point. The location of this point is determined by the rotation speed of the galactic bar.

When the researchers looked at the chemical makeup of stars, they discovered evidence of such an outward cosmic migration. The Hercules Stream stars are rich in heavier elements (called metals in astronomy), implying that they formed closer to the galactic center, where stars are about ten times more metal-rich than those in the galactic suburbs.

Co-author Dr. Ralph Schoenrich (UCL Physics & Astronomy) said in the statement: "The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter."

Schoenrich also added: "Our research provides a new type of measurement of dark matter ㅡ not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar's spin."

Dark matter is invisible and unknown in its nature, but astronomers believe that dark matter envelops the Milky Way – and other galaxies – in an enigmatic halo that extends far into space. According to scientists, the Universe is thought to contain approximately five times as much dark matter as ordinary, visible matter.

Alternative gravity theories, such as modified Newtonian dynamics, reject the concept of dark matter. They instead attempt to explain discrepancies by modifying Einstein's theory of general relativity.

"Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar," said co-author and Ph.D. student Rimpei Chiba of the University of Oxford.

Rimpei also added, "Our finding also poses a major problem for alternative gravity theories ㅡ as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar."