Massachusetts Institute of TechnologyExternal study shows milky way The stars rotate more slowly, indicating a lighter core containing less dark matter, which contradicts previous assumptions.
By recording the speed of stars throughout the Milky Way, MIT physicists found that stars in the galactic disk are moving slower than expected compared to stars closer to the galactic center. The results raise a surprising possibility: the Milky Way's gravitational core may be lighter, and contain less dark matter, than previously thought.
The new findings are based on the team's analysis of data captured by Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise position, distance and motion 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,” or how fast the star is rotating in the galactic disk, given the star's distance from the center of the galaxy. .
Understanding galactic rotation
Scientists plotted the speed of each star against its distance to generate a rotation curve, a standard graph in astronomy that represents how quickly matter rotates 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.
“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,” says Lina Naguib, 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.”
Challenging dark matter theories
The team translated the new rotation curve into a distribution of dark matter that could explain the slowing down of exostars, and found that the resulting map produced a lighter galaxy core than expected. That is, the center of the Milky Way may be less dense and contain less dark matter than scientists think.
“This puts this result in tension with other measurements,” says Najeeb. “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 team announces its results this month in Monthly Notices of the Journal of the Royal Society. Co-authors of the study from MIT, including Nassib, are first author Xiaowei Ou, Anna-Kristina Ehlers, and Anna Friebel.
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 notice that galaxies rotate in ways that cannot be driven by purely visible matter. She and her colleagues measured the rotational speed of stars, and found that the resulting rotation curves were surprisingly flat. This means that the speed of the stars remained the same throughout the galaxy, rather than decreasing with distance. They concluded that another type of invisible matter must be affecting distant stars to give them an extra boost.
Rubin's work on rotation curves was one of the first strong pieces of evidence for the existence of dark matter, an invisible and unknown entity estimated to outweigh all stars and other visible matter in the universe.
Since then, astronomers have observed similar flat curves in distant galaxies, supporting the existence of dark matter. Only recently have astronomers tried to plot the rotation curve of our galaxy with stars.
“It turns out that it's hard to measure the rotation curve when you're sitting inside a galaxy,” Au says.
New insights from Gaia data
In 2019, Anna Christina Ehlers, an assistant professor of physics at MIT, plotted the rotation curve of the Milky Way, using a previous set of data from the Gaia satellite. This data release included stars up to 25 kiloparsecs, or about 81,000 light-years, from the galactic center.
Based on these data, Ehlers observed that the Milky Way's rotation curve appears flat, albeit with a slight dip, similar to other distant galaxies, and by inference, the galaxy likely carries a high density of dark matter in its core. But that opinion has changed now, as the telescope has released a new set of data, this time including stars as distant as 30 kiloparsecs, roughly 100,000 light-years from the galactic core.
“At these distances, we are at the edge of the galaxy where stars begin to fade,” says Fripple. “No one has explored how matter moves in this outer galaxy, where we are literally in nothingness.”
Frebel, Naguib, Au, and Ehlers jumped into the new Gaia data, looking to expand on Ehlers' initial rotation curve. To improve their analysis, the team supplemented the Gaia data with measurements by APOGEE – Apache Point Observatory's Galaxy Evolution Experiment, which measures highly detailed properties of more than 700,000 stars in the Milky Way, such as their brightness, temperature and elemental composition.
“We feed all this information into an algorithm to try to learn connections that can then give us better estimates of the distance to the star,” Au explains. “This is how we can push further.”
The team determined the precise distances of more than 33,000 stars and used these measurements to create a 3D map of stars spread across the Milky Way out to about 30 kiloparsecs. They then combined this map into a model of circular velocity, simulating the speed at which any star should move, taking into account the distribution of all other stars in the galaxy. They then plotted each star's speed and distance on a chart to produce an updated rotation curve for the Milky Way.
“Here came the strangeness,” Naguib says.
Instead of seeing a slight dip like previous rotation curves, the team noticed that the new curve dropped more strongly than expected at the outer end. This unexpected dip indicates that while stars may travel at the same speed out to a certain distance, they suddenly slow down at farther distances. The stars in the suburbs appear to be moving more slowly than expected.
Explore the secrets of the galaxy
When the team translated this rotation curve into the amount of dark matter that should be present throughout the galaxy, they found that the core of the Milky Way may contain less dark matter than previously estimated.
“This result is inconsistent with other measurements,” says Najeeb. “Really understanding this result will have profound ramifications. This could lead to more hidden masses beyond the edge of the galactic disk, or a revision of the equilibrium state of our galaxy. We seek to find these answers in upcoming work, using high-resolution simulations of Milky Way-like galaxies .
Reference: “Dark matter picture of the Milky Way inferred from its circular velocity curve” by Xiaowei Ou, Anna-Kristina Ehlers, Lena Naguib, and Anna Friebel, 08 January 2024, Monthly Notices of the Royal Astronomical Society.
This research was funded in part by the National Science Foundation.
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