By clocking the speed of stars throughout the Milky Way galaxy, MIT physicists have found that stars further out within the galactic disk are traveling more slowly than expected in comparison with stars which might be closer to the galaxy’s center. The findings raise a surprising possibility: The Milky Way’s gravitational core could also be lighter in mass, and contain less dark matter, than previously thought.
The brand new results are based on the team’s evaluation of knowledge taken by the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise location, distance, and motion of greater than 1 billion stars throughout the Milky Way galaxy, while APOGEE is a ground-based survey. The physicists analyzed Gaia’s measurements of greater than 33,000 stars, including among the farthest stars within the galaxy, and determined each star’s “circular velocity,” or how briskly a star is circling within the galactic disk, given the star’s distance from the galaxy’s center.
The scientists plotted each star’s velocity against its distance to generate a rotation curve — a regular graph in astronomy that represents how briskly matter rotates at a given distance from the middle of a galaxy. The form of this curve may give scientists an idea of how much visible and dark matter is distributed throughout a galaxy.
“What we were really surprised to see was that this curve remained flat, flat, flat out to a certain distance, after which it began tanking,” says Lina Necib, assistant professor of physics at MIT. “This implies the outer stars are rotating somewhat slower than expected, which is a really surprising result.”
The team translated the brand new rotation curve right into a distribution of dark matter that might explain the outer stars’ slow-down, and located the resulting map produced a lighter galactic core than expected. That’s, the middle of the Milky Way could also be less dense, with less dark matter, than scientists have thought.
“This puts this lead to tension with other measurements,” Necib says. “There’s something fishy happening somewhere, and it’s really exciting to determine where that’s, to essentially have a coherent picture of the Milky Way.”
The team reports its results this month within the Monthly Notices of the Royal Society Journal. The study’s MIT co-authors, including Necib, are first writer Xiaowei Ou, Anna-Christina Eilers, and Anna Frebel.
“Within the nothingness”
Like most galaxies within the universe, the Milky Way spins like water in a whirlpool, and its rotation is driven, partially, by all of the matter that swirls inside its disk. Within the Nineteen Seventies, astronomer Vera Rubin was the primary to watch that galaxies rotate in ways that can not be driven purely by visible matter. She and her colleagues measured the circular velocity of stars and located that the resulting rotation curves were surprisingly flat. That’s, the rate of stars remained the identical throughout a galaxy, quite than dropping off with distance. They concluded that another kind of invisible matter should be acting on distant stars to offer them an added push.
Rubin’s work in rotation curves was one in all the primary strong pieces of evidence for the existence of dark matter — an invisible, unknown entity that’s estimated to outweigh all the celebs and other visible matter within the universe.
Since then, astronomers have observed similar flat curves in far-off galaxies, further supporting dark matter’s presence. Only recently have astronomers attempted to chart the rotation curve in our own galaxy with stars.
“It seems it’s harder to measure a rotation curve once you’re sitting inside a galaxy,” Ou notes.
In 2019, Anna-Christina Eilers, assistant professor of physics at MIT, worked to chart the Milky Way’s rotation curve, using an earlier batch of knowledge released by the Gaia satellite. That data release included stars as far out as 25 kiloparsecs, or about 81,000 light years, from the galaxy’s center.
Based on these data, Eilers observed that the Milky Way’s rotation curve seemed to be flat, albeit with mild decline, just like other far-off galaxies, and by inference, the galaxy likely bore a high density of dark matter at its core. But this view now shifted, because the telescope released a brand new batch of knowledge, this time including stars as far out as 30 kiloparsecs — almost 100,000 light years from the galaxy’s core.
“At these distances, we’re right at the sting of the galaxy where stars begin to peter out,” Frebel says. “Nobody had explored how matter moves around on this outer galaxy, where we’re really within the nothingness.”
Weird tension
Frebel, Necib, Ou, and Eilers jumped on Gaia’s latest data, trying to expand on Eilers’ initial rotation curve. To refine their evaluation, the team complemented Gaia’s data with measurements by APOGEE — the Apache Point Observatory Galactic Evolution Experiment, which measures extremely detailed properties of greater than 700,000 stars within the Milky Way, equivalent to their brightness, temperature, and elemental composition.
“We feed all this information into an algorithm to attempt to learn connections that may then give us higher estimates of a star’s distance,” Ou explains. “That is how we will push out to farther distances.”
The team established the precise distances for greater than 33,000 stars and used these measurements to generate a three-dimensional map of the celebs scattered across the Milky Way out to about 30 kiloparsecs. They then incorporated this map right into a model of circular velocity, to simulate how briskly anybody star should be traveling, given the distribution of all the opposite stars within the galaxy. They then plotted each star’s velocity and distance on a chart to provide an updated rotation curve of the Milky Way.
“That is where the weirdness got here in,” Necib says.
As an alternative of seeing a gentle decline like previous rotation curves, the team observed that the brand new curve dipped more strongly than expected on the outer end. This unexpected downturn suggests that while stars may travel just as fast out to a certain distance, they suddenly decelerate on the farthest distances. Stars on the outskirts appear to travel more slowly than expected.
When the team translated this rotation curve to the quantity of dark matter that must exist throughout the galaxy, they found that the Milky Way’s core may contain less dark matter than previously estimated.
“This result’s in tension with other measurements,” Necib says. “Really understanding this result could have deep repercussions. This might result in more hidden masses just beyond the sting of the galactic disk, or a reconsideration of the state of equilibrium of our galaxy. We seek to search out these answers in upcoming work, using high resolution simulations of Milky Way-like galaxies.”
This research was funded, partially, by the National Science Foundation.
