Stars at the edge of our Milky Way galaxy travel more slowly

Recent research by MIT physicists has led to an intriguing discovery regarding the motion of stars in the Milky Way galaxy.

Their study suggests a possible revision in our understanding of the galaxy’s composition, particularly in terms of its gravitational core and the elusive dark matter.

Cosmic tools Gaia and APOGEE

Utilizing data from the Gaia space telescope and the APOGEE ground-based survey, the team embarked on an extensive analysis of over 33,000 stars across the Milky Way.

Gaia, an orbiting telescope, offers precise tracking of the location, distance, and movement of stars, while APOGEE complements this with detailed ground observations.

The focal point of the study was the “circular velocity” of the stars — essentially, how swiftly each star orbits within the galactic disk relative to its distance from the galaxy’s center.

By plotting these velocities against their respective distances, the team constructed a rotation curve, a critical tool in astronomy for understanding the distribution of both visible and dark matter within a galaxy.

Odd behavior of the Milky Way’s outer stars

“What we were really surprised to see was that this curve remained flat, flat, flat out to a certain distance, and then it started tanking,” says Lina Necib, assistant professor of physics at MIT. “This means the outer stars are rotating a little slower than expected, which is a very surprising result.”

This unexpected behavior of the outer stars prompted the team to reinterpret the distribution of dark matter within the Milky Way, leading to a startling conclusion: the galaxy’s core might be lighter and contain less dark matter than previously believed.

“This puts this result in tension with other measurements,” Necib says. “There is something fishy going on somewhere, and it’s really exciting to figure out where that is, to really have a coherent picture of the Milky Way.”

The study features contributions from MIT scientists, including first author Xiaowei Ou, Anna-Christina Eilers, and Anna Frebel.

Vera Rubin’s and dark matter

The research builds on the pioneering work of astronomer Vera Rubin from the 1970s. Rubin first observed that galaxies rotate in a manner that cannot be solely attributed to visible matter.

Her work suggested that some unseen, or “dark,” matter exerted an influence on distant stars, accounting for their unexpected motion.

Rubin’s pioneering work in understanding galactic rotation curves laid the groundwork for the discovery of dark matter, an elusive entity that seemingly outweighs all visible matter in the universe.

Since then, astronomers have consistently found similar flat rotation curves in distant galaxies, reinforcing the theory of dark matter’s omnipresence.

Challenge of measuring Milky Way stars

However, charting the rotation curve of the Milky Way, our own galaxy, has presented unique challenges.

“It turns out it’s harder to measure a rotation curve when you’re sitting inside a galaxy,” notes Xiaowei Ou.

This challenge was undertaken in 2019 by Anna-Christina Eilers, assistant professor of physics at MIT, using data from the Gaia satellite.

Gaia’s initial data release, which included stars up to 25 kiloparsecs (approximately 81,000 light years) from the galaxy’s center, suggested a flat yet mildly declining rotation curve for the Milky Way, hinting at a high concentration of dark matter at its core.

A more recent batch of Gaia data, encompassing stars up to 30 kiloparsecs (nearly 100,000 light years) from the core, offered new insights.

Anna Frebel points out the significance of these distances, “We’re at the galaxy’s edge, where stars become sparse, and understanding matter movement here is exploring the unknown.”

Advanced technological tools

Leveraging this new data, Frebel, Necib, Ou, and Eilers sought to build upon Eilers’ initial findings.

They enhanced Gaia’s data with APOGEE (Apache Point Observatory Galactic Evolution Experiment) measurements, which provide intricate details on over 700,000 Milky Way stars, including 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 a star’s distance,” Ou explains. “That’s how we can push out to farther distances.”

The team determined precise distances for over 33,000 stars, creating a three-dimensional map of the Milky Way up to 30 kiloparsecs.

Pushing the boundaries of star measurement

They used this map to model circular velocity, simulating each star’s travel speed based on the distribution of all other stars.

Plotting each star’s velocity and distance, they produced an updated rotation curve.

Here, the unexpected emerged. The curve showed a more significant dip than expected at the outer end. Contrary to earlier findings, stars at the galaxy’s outskirts travel slower than anticipated.

This downturn suggests that while stars maintain speed up to a certain point, they abruptly slow down in the farthest reaches.

Dark matter and the Milky Way’s outer stars

Translating this rotation curve to the requisite amount of dark matter, the team inferred that the Milky Way’s core might house less dark matter than previously thought.

“This result is in tension with other measurements,” Necib says. “Really understanding this result will have deep repercussions. This might lead to more hidden masses just beyond the edge of the galactic disk, or a reconsideration of the state of equilibrium of our galaxy. We seek to find these answers in upcoming work, using high resolution simulations of Milky Way-like galaxies.”

In summary, this study’s conclusions have significantly altered our understanding of the Milky Way. By employing sophisticated techniques and advanced tools like the Gaia satellite and APOGEE, they have revealed a surprising slowdown in the rotation speeds of stars at the galaxy’s outskirts, challenging previous assumptions about the distribution of dark matter in the Milky Way.

Their research underscores the dynamic nature of space exploration and raises new questions for future investigations. It prompts a reevaluation of existing galactic models and propels the scientific community towards uncovering the deeper mysteries of our universe, particularly in understanding the elusive nature of dark matter.

The full study was published in the journal Monthly Notices of the Royal Astronomical Society.

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