By timing the speeds of stars throughout the Milky Way, MIT physicists have discovered that stars further out in the Milky Way's disk are moving slower than expected compared to stars closer to the center. The discovery raises the surprising possibility that the Milky Way's gravitational core may be lighter and contain less dark matter than previously thought.
The new results are based on the team's analysis of data obtained by the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise position, distance and movement of more than 1 billion stars throughout the Milky Way, while APOGEE is a ground-based measurement instrument. Physicists analyzed Gaia's measurements of more than 33,000 stars, including some of the most distant stars in the Milky Way, and determined each star's "circular velocity," which is how fast a star spirals around the Milky Way's disk, taking into account the star's distance from the galactic center.
Understanding the rotation of the Milky Way
The scientists plotted each star's speed against its distance into a rotation curve -- a standard graph in astronomy that represents how fast matter spins at a certain distance from the center of a galaxy. The shape of this curve allows scientists to understand the distribution of visible and dark matter throughout the galaxy.
"We were very surprised to find that this curve stays flat, flat, flat for a certain distance and then starts to slide down," said Lena Nechib, an assistant professor of physics at MIT. "This means that the outer star is rotating a little slower than expected, which is a very surprising result."
Challenging dark matter theory
The team translated the new rotation curve into a distribution of dark matter, which explains the slowing down of outer stars, and found that the resulting map of the galactic core is lighter than expected. In other words, the density of the center of the Milky Way may be less than scientists thought, and there may be less dark matter than imagined.
"This puts this result in conflict with other measurements," Necib said. "There has to be something fishy somewhere, and it's exciting to find out where it is and really understand the full picture of the Milky Way."
The research team reported their findings this month in the Monthly Notices of the Royal Society. Including Negib, MIT co-authors of the study include first author Xiaowei Ou, Anna-Christina Ehlers and Anna Frebel.
"In nothingness"
Like most galaxies in the universe, the Milky Way spins like water in a vortex, its rotation driven in part by all the matter swirling within its disk. In the 1970s, astronomer Vera Rubin was the first to observe that the rotation of galaxies cannot be driven purely by visible matter. She and her colleagues measured the star's circular velocity and found that the rotation curve was surprisingly flat. That is, the star's speed remains constant throughout the galaxy, rather than decreasing with distance. They concluded that there must be other types of invisible matter acting on distant stars, giving them an added push.
Rubin's work on rotation curves was one of the first strong evidences for the existence of dark matter, an invisible, unknown entity that is estimated to outnumber all stars and other visible matter in the universe.
Since then, astronomers have observed similar flat curves in distant galaxies, further proving the existence of dark matter. Until recently, astronomers had tried to map the rotation curve of our Milky Way using stars. It turns out that obtaining the rotation curve is more difficult when the surveyor is sitting inside the galaxy.
New revelations brought by Gaia data
In 2019, Anna-Christina Eilers, an assistant professor of physics at MIT, used a batch of data released earlier by the Gaia satellite to plot the rotation curve of the Milky Way. The data released this time includes stars as far away as 25 kilopascals (about 81,000 light-years) from the center of the Milky Way.
Based on these data, Ehlers observed that the Milky Way's rotation curve appears to be flat, albeit with a slight dip, similar to that of other distant galaxies, inferring that the Milky Way's core likely contains a high density of dark matter. But that view has now changed, as the telescope has released a new batch of data, this time including stars nearly 100,000 light-years away from the Milky Way's core.
strange tension
The team supplemented the Gaia data with measurements from the Apache Point Observatory Galactic Evolution Experiment (APOGEE). APOGEE has measured extremely detailed properties, such as brightness, temperature and elemental composition, of more than 700,000 stars in the Milky Way.
The team determined the precise distances of more than 33,000 stars and used these measurements to generate a three-dimensional map of stars scattered across the Milky Way within about 30 kilopascals. They then incorporated this map into a circular velocity model, simulating how fast any one star would travel given the distribution of all other stars in the galaxy. They then plotted each star's speed and distance on a graph to map out the Milky Way's latest rotation curve.
Instead of seeing a slight dip like the previous rotational curve, the team observed that the new curve dropped more sharply on the outside than expected. This unexpected dip suggests that while stars may be traveling just as fast at certain distances, at their greatest distances they suddenly slow down. Stars located on the outskirts appear to move slower than expected.
When the team converted this rotation curve into the amount of dark matter that must be present throughout the Milky Way, they found that the Milky Way's core may contain less dark matter than previously estimated.
This result is contradictory to other measurements. Truly understanding this outcome will have far-reaching consequences. This could lead to more mass hiding beyond the edge of the galactic disk, or to reconsider the equilibrium state of our galaxy. We hope to find these answers in future work using high-resolution simulations similar to the Milky Way.
Compiled source: ScitechDaily