Galaxy’s core may contain less dark matter than expected

By measuring the speed of stars throughout the Milky Way, MIT physicists found that stars located farther out in the galactic disk move more slowly than expected compared to stars closer to the galaxy’s center. The results raise a surprising possibility: The Milky Way’s gravitational core could be lighter in mass and contain less dark matter than previously thought.

The new results are based on the team’s analysis of data taken by the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise location, distance and movement of more than a billion stars throughout the Milky Way, while APOGEE is a ground-based survey.

Physicists analyzed Gaia’s measurements of more than 33,000 stars, including some of the most distant stars in the galaxy, and determined each star’s “circular velocity,” that is, how fast a star rotates in the galactic disk, taking into account the distance between the star and the center of the galaxy. .

The scientists plotted each star’s speed versus its distance to generate a rotation curve, a standard graph in astronomy that represents the rate at which matter rotates at a given distance from the center of a galaxy. The shape of this curve can give scientists an idea of ​​how much visible and dark matter is distributed in a galaxy.

“What really surprised us was that this curve stayed flat, flat, flat until a certain distance away, and then it started to collapse,” says Lina Necib, an assistant professor of physics at MIT. “This means that the outer stars are rotating a little slower than expected, which is a very surprising result.”

The team translated the new rotation curve into a distribution of dark matter that could explain the slowing of outer stars, and found that the resulting map produced a lighter-than-expected galactic core. In other words, the center of the Milky Way could be less dense and contain less dark matter than scientists thought.

“This puts this result in tension with other measurements,” explains Necib. “There’s something fishy going on somewhere, and it’s really exciting to figure out where it is, to have a really coherent picture of the Milky Way.”

The team reports its results in the Monthly Notices of the Royal Astronomical Society. Co-authors of the study at MIT, including Necib, are first authors Xiaowei Ou, Anna-Christina Eilers and Anna Frebel.

‘Into nothingness’

Like most galaxies in the universe, the Milky Way spins like water in a whirlpool, and its rotation is driven in part by all the matter swirling inside its disk. In the 1970s, astronomer Vera Rubin was the first to observe that galaxies rotate in ways that cannot be driven by visible matter alone.

She and her colleagues measured the circular speed of stars and found that the resulting rotation curves were surprisingly flat. In other words, the speed of stars remained the same throughout a galaxy, rather than decreasing with distance. They concluded that another type of invisible matter must be acting on distant stars to give them an extra boost.

Rubin’s work on rotation curves was one of the first solid proofs of the existence of dark matter, an invisible and unknown entity that is estimated to exceed all stars and other visible matter in the universe .

Since then, astronomers have observed similar flat curves in distant galaxies, confirming the presence of dark matter. Only recently have astronomers attempted to trace the rotation curve of stars in our own galaxy.

“It turns out that it’s more difficult to measure a rotation curve when you’re sitting inside a galaxy,” Ou notes.

In 2019, Anna-Christina Eilers, assistant professor of physics at MIT, worked to plot the rotation curve of the Milky Way, using a previous batch of data published by the Gaia satellite. This data release included stars located up to 25 kiloparsecs, or about 81,000 light years, from the center of the galaxy.

Based on these data, Eilers observed that the rotation curve of the Milky Way appeared to be flat, although with a slight decay, similar to that of other distant galaxies, and by inference the galaxy probably contained a strong density of dark matter at its core. But that view has now changed when the telescope released a new batch of data, this time including stars up to 30 kiloparsecs, or nearly 100,000 light years, from the galaxy’s core.

“At these distances, we are right at the edge of the galaxy, where the stars start to disappear,” says Frebel. “No one had explored how matter moves in this outer galaxy, where we are really in nothingness.”

Strange tensions

Frebel, Necib, Ou and Eilers jumped on Gaia’s new data, looking to extend Eilers’ initial rotation curve. To refine their analysis, the team supplemented the Gaia data with measurements from APOGEE, the Apache Point Observatory’s Galactic Evolution Experiment, which measures the extremely detailed properties of more than 700,000 stars in the Pathway. milky, such as their luminosity, temperature and elemental composition.

“We feed all of this information into an algorithm to try to learn connections that can then give us better estimates of a star’s distance,” says Ou. “This is how we can reach greater distances.”

The team established 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 out to about 30 kiloparsecs. They then incorporated this map into a circular velocity model, to simulate how fast a star must move, given the distribution of all the other stars in the galaxy. They then plotted the speed and distance of each star on a graph to produce an updated rotation curve of the Milky Way.

“That’s where the weirdness came in,” Necib says.

Instead of seeing a slight drop like previous rotation curves, the team observed that the new curve dropped more steeply than expected at the outer end. This unexpected slowdown suggests that although stars can move just as quickly over a certain distance, they suddenly slow down over farther distances. Stars on the outskirts appear to be traveling more slowly than expected.

When the team translated this rotation curve into the amount of dark matter that must exist throughout the galaxy, they discovered that the core of the Milky Way might contain less dark matter than previously estimated.

“This result is in contradiction with other measurements,” explains Necib. “A true understanding of this result will have profound implications. It could lead to the presence of more hidden masses just beyond the edge of the galactic disk, or to a reconsideration of the equilibrium state of our galaxy. We seek to find these answers in future work, using high-precision data-resolution simulations of galaxies similar to the Milky Way.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.