Stars have been spotted creeping around the edges of the Milky Way Galaxy more slowly than expected, a slow motion that scientists say can only be explained if our galaxy’s map of dark matter is wrong.
Specific velocities stars Around the edges of galaxies were historically dead giveaways of existence Dark matter In those galaxies. This is because astronomers can measure the galaxy’s “rotation curve,” which plots the orbital velocities of stars against their distances from the galactic center.
If dark matter (and hence the gravitational influence it provides) did not exist, stars would begin to slow down as they move away from the galactic center. But instead, in the 1960s and early 1970s, astronomers discovered… Vera Rubin Kent Ford noticed that the rotation curves of galaxies were flat. In other words, the orbital motion of stars does not decrease with distance. They kept the pace up. Scientists believe that the explanation for this is that galaxies are confined within halos of dark matter. These halos are thought to be densest in the center of the galaxy; that it gravity It is this dark matter that makes stars move.
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But here’s the thing – because we sit inside our galaxy, and we lack a bird’s-eye view of it, measuring the rotation curve of our galaxy. milky way It proved more difficult.
What we need is accurate distance information so we can know how far distant stars are from the center of the galaxy. In 2019, Anna Cristina Ehlers of the Massachusetts Institute of Technology (MIT) led a research team that used… European Space AgencyMeasure the stars Gaia mission To plot the orbital velocities of stars to 80,000 Light year From the center of the galaxy. As expected, the researchers found a flat rotation curve with only a slight hint of a decrease in velocity for the outer stars in that sample.
However, new results combining Gaia measurements with those of the APOGEE experiment (Apache Point Observatory for Galaxy Evolution Experiment), which was performed on a ground-based telescope in New Mexico, USA, and which measures the physical properties of stars to better judge their distance, has already achieved new results. He measured the Milky Way’s rotation curve of stars farther away than ever before, out to about 100,000 light-years.
“What we were really surprised to see was that this curve stayed flat, flat, flat for a certain distance, and then it started to decline,” Salaina Naguib, an assistant professor of physics at MIT, said in an article. statement. “This means that the outer stars are rotating a little slower than expected, which is a very surprising result.”
“At these distances, we are at the edge of the galaxy where stars begin to fade,” Anna Frebel of MIT added in the same statement. “No one has explored how matter moves in this outer galaxy, where we are truly in nothingness.”
The decrease in orbital velocity at these distances means that there is less Dark matter in the center of our galaxy is greater than expected. The research team describes the galaxy’s halo of dark matter as “pitted,” somewhat like an apple. The crew also says there isn’t enough gravity for the dark matter that appears to be there to reach a distance of 100,000 light-years and keep the stars moving at the same speed.
“This puts this result in tension with other measurements,” Naguib said. “There’s something fishy going on somewhere, and it’s really interesting to find out where that is, to get a coherent picture of the Milky Way.”
The next step, Naguib says, is to use high-resolution computer simulations to model different distributions of dark matter within our galaxy to see which best replicates the downward rotation curve. Models of Galaxy formation It can then be attempted to explain how the Milky Way arrived at its specific, specific distribution of dark matter, and why other galaxies did not.
The results were published on January 8 in the journal Monthly Notices of the Royal Astronomical Society.