Massachusetts Institute of Technologyresearch shows outside milky way The star’s slower rotation indicates a lighter core with less dark matter, contrary to previous assumptions.
By measuring the speed of stars across the Milky Way galaxy, MIT physicists found that stars outside the galactic disk are moving more slowly than expected compared to stars closer to the galaxy’s center. discovered. The discovery raises a surprising possibility: The Milky Way’s gravitational core may be less massive and contain less dark matter than previously thought.
The new results are based on the team’s analysis of data acquired by Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise location, distance, and movement of her more than 1 billion stars across the Milky Way galaxy, while APOGEE is a ground-based survey. Physicists analyzed Gaia’s measurements of more than 33,000 stars, including some in the galaxy’s farthest reaches, and calculated each star’s “circumferential velocity,” or distance from the center of the galaxy. determined how fast it rotates within the galactic disk. .
Understanding the rotation of galaxies
Scientists created rotation curves by plotting the speed of each star versus its distance. A rotation curve is a standard graph in astronomy that shows how fast matter rotates at a given distance from the center of a galaxy. The shape of this curve allows scientists to determine how much visible and dark matter is distributed throughout the galaxy.
“What we were really surprised to see was that this curve stayed flat, flat, flat up to a certain distance, and then it started to slope,” said Lina Necib, assistant professor of physics at MIT. says. “This means that the outer star is rotating a little more slowly than expected, which is a very surprising result.”
Challenging dark matter theory
The researchers translated the new rotation curves 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. This means the Milky Way’s center may be less dense and contain less dark matter than scientists thought.
“This makes this result inconsistent with other measurements,” Necib says. “There’s something fishy going on somewhere, and it’s really fun to figure out where it is and get consistent pictures of the Milky Way.”
The team will report its results this month. Royal Society Journal Monthly Notices. His MIT co-authors on this study, including Necib, are lead authors Xiaowei Ou, Anna-Christina Ailers, and Anna Frebel.
“In the void”
Like most galaxies in the universe, the Milky Way spins like water in a vortex, and its rotation is caused in part by all the matter swirling around in its disk. In the 1970s, astronomer Vera Rubin first observed that galaxies rotate in ways that cannot be driven by visible matter alone. She and her colleagues measured the star’s circumferential velocity and found that the resulting rotation curve was surprisingly flat. That is, the speed of the stars remained the same throughout the galaxy, rather than decreasing with distance. They concluded that some other type of invisible material must be acting on the distant star, giving it an additional thrust.
Rubin’s work on rotation curves was one of the first strong pieces of evidence for the existence of dark matter. Dark matter is an unknown, invisible entity estimated to be heavier than all the stars and other visible matter in the universe.
Since then, astronomers have observed similar flat curves in distant galaxies, further supporting the existence of dark matter. Only recently have astronomers attempted to graph the rotation curves of the stars in our galaxy.
“We found that it’s difficult to measure rotation curves when you’re sitting inside a galaxy,” Oh points out.
New insights from Gaia Data
In 2019, Anna Christina Eilers, an assistant professor of physics at MIT, set out to map the rotation curve of the Milky Way galaxy using an early set of data released by the Gaia satellite. The data release included stars as far away as 25 kiloparsecs, or about 81,000 light-years, from the center of the galaxy.
Based on these data, Eilers observed that the Milky Way’s rotation curve, like other distant galaxies, appears to be flat, albeit with a mild decline, and by inference, this galaxy We thought that there is a high possibility that it contains dense dark matter at its center. However, this view has changed after this telescope released a new set of data. This time, it included stars as far away as 30 kiloparsecs, almost 100,000 light-years from the center of the galaxy.
“At this distance, we are at the edge of the galaxy where stars start to decline,” Froebel says. “No one had ever investigated how matter moves in this outer galaxy where we actually have nothing.”
strange tension
Frebel, Necib, Ou, and Eilers jumped on Gaia’s new data and sought to extend Eilers’ initial rotation curve. To refine their analysis, the team supplemented Gaia’s data with measurements from his APOGEE (Apache Point Observatory’s Galactic Evolution Experiment). APOGEE will measure highly detailed properties of more than 700,000 stars in the Milky Way, including their brightness, temperature, and elemental composition.
“We feed all this information into an algorithm that tries to learn connections that allow us to more accurately estimate the distances of stars,” Ou explains. “That allows you to push further.”
The research 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 down to about 30 kiloparsecs. They then incorporated this map into a model of circular velocity, simulating how fast a single star would have to travel given the distribution of all the other stars in the galaxy. They then plotted each star’s velocity and distance on a chart to create an updated rotation curve for the Milky Way.
“That’s where something strange happened,” Necib says.
Rather than seeing a gradual decline like the previous rotation curve, the team observed that the new curve had a stronger decline than expected at the outer edges. This unexpected slump suggests that the stars move at the same speed up to a certain distance, but suddenly slow down at the furthest distances. The stars in the suburbs appear to be moving more slowly than expected.
Explore the mysteries of the galaxy
When the researchers translated this rotation curve into the amount of dark matter that would be present in the entire galaxy, they found that the Milky Way’s core may contain less dark matter than previously estimated.
“This result is inconsistent with other measurements,” Necib says. “Really understanding this result would have profound implications. This could mean that there are more hidden clumps just beyond the edge of the galactic disk, or that it could lead to a reconsideration of the galaxy’s equilibrium state.” We will try to find these answers in future studies using high-resolution simulations of galaxies similar to the Milky Way.”
Reference: “Dark matter profile of the Milky Way inferred from the circular velocity curve” Xiaowei Ou, Anna-Christina Ailers, Lina Necib, Anna Frebel, January 8, 2024, Royal Astronomical Society Monthly Notices.
DOI: 10.1093/mnras/stae034
This research was funded in part by the National Science Foundation.