Astrophysicists have predicted such a slowdown for 30 years, but this is the first time it has been measured. According to the researchers, it provides new insight into the nature of dark matter, which acts as a counterweight slowing the spin.
According to a new study by researchers at University College London (UCL) and the University of Oxford, the spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation.
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 are in resonance with the bar, meaning they revolve around the galaxy at the same rate as the bar’s spin.
These new discoveries not only shed light on the rotation of the Milky Way, but also on the nature of one of the universe’s most elusive materials — dark matter.
The Milky Way rotates at a whopping 130 miles (210 kilometers) per second, but a new study has found that dark matter has slowed the rotation of its bar by at least 24% since its formation nearly 14 billion years ago.
The spinning bar has gravitationally trapped these stars. The same thing happens when Jupiter’s Trojan and Greek asteroids orbit Jupiter’s Lagrange points (ahead and behind Jupiter). If the bar’s spin slows, these stars should move further out in the galaxy, keeping their orbital period in sync with the bar’s spin.
The stars in the stream have a chemical fingerprint: they are richer in heavier elements (called metals in astronomy), indicating that they have traveled away from the galactic center, where stars and star-forming gas are about ten times as rich in metals as the outer galaxy.
Using this information, the team calculated that the bar, which is made up of billions of stars and trillions of solar masses, has slowed its spin by at least 24 percent since its formation.
The Milky Way is a barred spiral galaxy with a dense band of stars in the center and large pivoting arms stretching out across the universe. Scientists believe that the Milky Way is surrounded by a halo of dark matter that extends far beyond its visible edge, as seen in other galaxies.
“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,” said co-author Dr. Ralph Schoenrich (UCL Physics & Astronomy).
Dark matter must be the counterweight slowing this spin. We could only infer dark matter until now by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter. Our findings provide a new way to measure dark matter – not in terms of gravitational energy, but of inertial mass (the dynamical response), which slows the bar’s spin.
“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. Our discovery also poses a major problem for alternative gravity theories, as they predict no, or significantly too little, slowing of the bar because they lack dark matter in the halo.”
Like other galaxies, the Milky Way is thought to be surrounded by a halo of dark matter that extends far beyond its visible edge. Dark matter is invisible and unknown in its nature, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see. 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 idea of dark matter, instead of seeking to explain discrepancies by tweaking Einstein’s theory of general relativity. The Milky Way galaxy is a barred spiral galaxy, with a thick bar of stars in the center and spiral arms extending through the disc beyond the bar. The bar rotates in the same way that the galaxy does.
The Royal Society, the Takenaka Scholarship Foundation, and the Science and Technology Facilities Council all provided funding for the study (STFC).
The study introduces a new method for calculating the inertial mass of dark matter. The Milky Way, according to researchers, is surrounded by a “halo” of dark matter that extends far beyond the visible edge of the galaxy.
