By measuring the speeds of stars in the Milky Way galaxy, MIT physicists found that the more distant stars of the galactic disk move slower than expected compared to stars closer to the center of the galaxy. The data reveal a surprising possibility: 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 with the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise position, distance and motion of more than 1 billion stars in the Milky Way galaxy; APOGEE is a place-based research.

Physicists analyzed Gaia measurements of more than 33,000 stars, including some of the most distant stars in the galaxy, and determined each star’s “circular velocity,” or the rate at which the star rotates in the galactic disk, taking into account the star’s distance from the galaxy. galactic center. .

Scientists plotted each star’s speed against its distance to create a spin curve, a standard astronomy graph that plots the spin rate of matter at a given distance from the center of the galaxy. The shape of this curve can give scientists an idea of ​​how much visible and dark matter is distributed throughout the galaxy.

“We were very surprised to see that this curve remained flat, flat, flat for a certain distance, and then started to drop,” says Lina Nesib, a professor of physics at MIT. “This means that outer stars rotate slightly slower than expected, which is a very surprising result.”

The team estimated the new spin curve based on the distribution of dark matter; This could explain the slowing down of outer stars, and found that the resulting map produced a lighter galactic core than expected. So the center of the Milky Way may be less dense and have less dark matter than scientists thought.

“This puts this result at odds with other measurements,” says Nesib. “There’s something weird going on somewhere, and it’s very interesting to figure out where that is to get a really coherent picture of the Milky Way.”

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

“In your absence”

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 realize that galaxies rotate in ways that cannot be controlled by visible matter alone.

He and his colleagues measured the stars’ circular velocity and found that the resulting rotation curves were surprisingly flat. That is, the speed of the stars did not change throughout the galaxy and did not decrease as distance increased. They concluded that another type of invisible matter must be acting to provide extra support to distant stars.

Rubin’s work on spin curves was one of the first convincing arguments for the existence of dark matter, an invisible, unknown entity estimated to outnumber all stars and other visible matter in the universe.

Since then, astronomers have observed similar smooth curves in distant galaxies, further confirming the existence of dark matter. Only recently have astronomers attempted to plot the rotation curve in our own stellar galaxy.

“It turns out that measuring the spin curve while sitting inside the galaxy is more difficult,” Ou says.

In 2019, Anna-Christina Eilers, a professor of physics at the Massachusetts Institute of Technology, worked to plot the rotation curve of the Milky Way using previous data broadcast by the Gaia satellite. This data release included stars within 25 kiloparsecs, or about 81,000 light-years, from the galactic center.

Based on these data, Eulers realized that the Milky Way’s rotation curve appeared flat, albeit with a slight tilt, similar to other distant galaxies, and based on this, the galaxy’s core likely contained a high density of dark matter. But that view changed as the telescope released a new dataset, this time including stars as far away as 30 kiloparsecs, about 100,000 light-years from the galactic core.

“At these distances, we are right at the edge of the galaxy where stars are starting to die,” says Froebel. “No one has actually studied how matter moves in this outer galaxy where we are extinct.”

strange tension

Froebel, Nesib, Ou, and Eulers used the new Gaia data to extend the initial Eulers return curve. To develop their analysis, the team supplemented the Gaia data with measurements from Apache Point Observatory’s Galactic Evolution Experiment APOGEE, which measures extremely detailed properties such as brightness, temperature, and elemental composition of more than 700,000 stars in the Milky Way.

“We feed all this information into an algorithm to try to learn the relationships, which can give us better estimates of the distance of the star,” Ou explains. “We can go longer distances this way.”

The team determined the exact distances of more than 33,000 stars and used these measurements to create a three-dimensional map of stars scattered across the Milky Way at a distance of about 30 kiloparsecs. They then added this map to the circular velocity model to simulate how fast a single star would have to be moving, given the distribution of all other stars in the galaxy. They then plotted the speed and distance of each star to obtain an updated rotation curve of the Milky Way.

“That’s where the miracle happened,” says Nesib.

The team noticed that instead of seeing a moderate decline as in previous return curves, the new curve showed more declines than expected at the outer end. This unexpected drop shows that although stars can travel a certain distance at the same speed, they suddenly slow down when they reach their furthest distances. Stars at the edges appear to be moving slower than expected.

When the team translated this rotation curve into the amount of dark matter that should be present in the galaxy, they found that the Milky Way’s core may contain less dark matter than previously thought.

“This result contradicts other measurements,” says Nesib. “A true understanding of this result will have profound implications. It could lead to the emergence of more hidden mass outside the galactic disk or to a revision of the equilibrium state of our galaxy. We aim to find these answers in future studies using high-resolution simulations of galaxies similar to the Milky Way.”