By clocking the pace of stars all through the Milky Means galaxy, MIT physicists have discovered that stars additional out within the galactic disk are touring extra slowly than anticipated in comparison with stars which can be nearer to the galaxy’s middle. The findings increase a shocking chance: The Milky Means’s gravitational core could also be lighter in mass, and comprise much less darkish matter, than beforehand thought.
The brand new outcomes are based mostly on the crew’s evaluation of knowledge taken by the Gaia and APOGEE devices. Gaia is an orbiting area telescope that tracks the exact location, distance, and movement of greater than 1 billion stars all through the Milky Means galaxy, whereas APOGEE is a ground-based survey. The physicists analyzed Gaia’s measurements of greater than 33,000 stars, together with among the farthest stars within the galaxy, and decided every star’s “round velocity,” or how briskly a star is circling within the galactic disk, given the star’s distance from the galaxy’s middle.
The scientists plotted every star’s velocity in opposition to its distance to generate a rotation curve — a normal 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 thought of how a lot seen and darkish matter is distributed all through a galaxy.
“What we had been actually shocked to see was that this curve remained flat, flat, flat out to a sure 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 anticipated, which is a really shocking end result.”
The crew translated the brand new rotation curve right into a distribution of darkish matter that would clarify the outer stars’ slow-down, and located the ensuing map produced a lighter galactic core than anticipated. That’s, the middle of the Milky Means could also be much less dense, with much less darkish matter, than scientists have thought.
“This places this end in stress with different measurements,” Necib says. “There’s something fishy happening someplace, and it’s actually thrilling to determine the place that’s, to actually have a coherent image of the Milky Means.”
The crew stories its outcomes this month within the Month-to-month Notices of the Royal Society Journal. The examine’s MIT co-authors, together with Necib, are first creator Xiaowei Ou, Anna-Christina Eilers, and Anna Frebel.
“Within the nothingness”
Like most galaxies within the universe, the Milky Means spins like water in a whirlpool, and its rotation is pushed, partly, by all of the matter that swirls inside its disk. Within the Seventies, astronomer Vera Rubin was the primary to look at that galaxies rotate in methods that can’t be pushed purely by seen matter. She and her colleagues measured the round velocity of stars and located that the ensuing rotation curves had been surprisingly flat. That’s, the speed of stars remained the identical all through a galaxy, relatively than dropping off with distance. They concluded that another sort of invisible matter have to be performing on distant stars to present them an added push.
Rubin’s work in rotation curves was one of many first robust items of proof for the existence of darkish matter — an invisible, unknown entity that’s estimated to outweigh all the celebrities and different seen matter within the universe.
Since then, astronomers have noticed related flat curves in far-off galaxies, additional supporting darkish matter’s presence. Solely lately have astronomers tried to chart the rotation curve in our personal galaxy with stars.
“It seems it’s more durable to measure a rotation curve whenever you’re sitting inside a galaxy,” Ou notes.
In 2019, Anna-Christina Eilers, assistant professor of physics at MIT, labored to chart the Milky Means’s rotation curve, utilizing an earlier batch of knowledge launched by the Gaia satellite tv for pc. That information launch included stars as far out as 25 kiloparsecs, or about 81,000 mild years, from the galaxy’s middle.
Based mostly on these information, Eilers noticed that the Milky Means’s rotation curve seemed to be flat, albeit with gentle decline, much like different far-off galaxies, and by inference, the galaxy possible bore a excessive density of darkish matter at its core. However this view now shifted, because the telescope launched a brand new batch of knowledge, this time together with stars as far out as 30 kiloparsecs — virtually 100,000 mild years from the galaxy’s core.
“At these distances, we’re proper on the fringe of the galaxy the place stars begin to peter out,” Frebel says. “Nobody had explored how matter strikes round on this outer galaxy, the place we’re actually within the nothingness.”
Bizarre stress
Frebel, Necib, Ou, and Eilers jumped on Gaia’s new information, seeking to increase on Eilers’ preliminary rotation curve. To refine their evaluation, the crew complemented Gaia’s information with measurements by APOGEE — the Apache Level Observatory Galactic Evolution Experiment, which measures extraordinarily detailed properties of greater than 700,000 stars within the Milky Means, equivalent to their brightness, temperature, and elemental composition.
“We feed all this info into an algorithm to attempt to study connections that may then give us higher estimates of a star’s distance,” Ou explains. “That’s how we are able to push out to farther distances.”
The crew established the exact distances for greater than 33,000 stars and used these measurements to generate a three-dimensional map of the celebrities scattered throughout the Milky Means out to about 30 kiloparsecs. They then included this map right into a mannequin of round velocity, to simulate how briskly anybody star have to be touring, given the distribution of all the opposite stars within the galaxy. They then plotted every star’s velocity and distance on a chart to provide an up to date rotation curve of the Milky Means.
“That’s the place the weirdness got here in,” Necib says.
As an alternative of seeing a gentle decline like earlier rotation curves, the crew noticed that the brand new curve dipped extra strongly than anticipated on the outer finish. This sudden downturn means that whereas stars might journey simply as quick out to a sure distance, they instantly decelerate on the farthest distances. Stars on the outskirts seem to journey extra slowly than anticipated.
When the crew translated this rotation curve to the quantity of darkish matter that should exist all through the galaxy, they discovered that the Milky Means’s core might comprise much less darkish matter than beforehand estimated.
“This result’s in stress with different measurements,” Necib says. “Actually understanding this end result could have deep repercussions. This may result in extra hidden lots simply past the sting of the galactic disk, or a reconsideration of the state of equilibrium of our galaxy. We search to search out these solutions in upcoming work, utilizing excessive decision simulations of Milky Means-like galaxies.”
This analysis was funded, partly, by the Nationwide Science Basis.
