New stars found in the Milky Way were born outside of it

(CNN)Astronomers have found a new stream of stars in our Milky Way galaxy, but they likely didn't originate there, according to a new study.

This new star cluster has been named Nyx, for the Greek goddess of the night. They are within the vicinity of our sun's location in the galaxy and extend about 6,000 light-years above and below the plane of the Milky Way galaxy if you were to view it from the side.

The study published this week in the journal Nature Astronomy.

The cluster of 250 stars are rotating with the Milky Way's galactic disk, where most of the galaxy's stars are located. But the Nyx stars are also moving toward the center of the galaxy.

"The two possible explanations here are that they are the remnants of a [galactic] merger, or that they are disk stars that got shaken into their new orbits because of a collision with the disk of the Milky Way," said Lina Necib, study author and postdoctoral scholar in theoretical physics at the California Institute of Technology, in an email to CNN.

Mergers are how galaxies grow. They gobble up smaller neighboring dwarf galaxies. Nyx was likely once a dwarf galaxy that merged with the Milky Way.

"If Nyx is indeed a dwarf galaxy, it would have collided with the disk of the Milky Way at a low [angle]," Necib said. "What makes Nyx special is its 'prograde' orbits, the fact that [the stars] are co-rotating with the disk. We have not detected this type of merger before."

A new way to study galaxies

Necib and her colleagues used a combination of supercomputers and the Gaia space observatory, which was launched by the European Space Agency in 2013, to find Nyx.

She studies the movement of stars and dark matter in the Milky Way, which can tell us a lot about our galaxy.

"If there are any clumps of stars that are moving together in a particular fashion, that usually tells us that there is a reason that they're moving together," she said.

Since Gaia was launched, the space observatory has been mapping a billion stars in our galaxy and beyond it, with the goal of creating an incredibly precise 3D map by mission's end in 2022.

And Caltech is just one of many institutions where researchers are dedicated to creating detailed simulations of galaxies that include all current information about galactic formation and evolution through the FIRE project, or Feedback in Realistic Environments. This project allows researchers to simulate the formation and evolution of galaxies over time.

"Galaxies form by swallowing other galaxies," Necib said. "We've assumed that the Milky Way had a quiet merger history, and for a while it was concerning how quiet it was because our simulations show a lot of mergers. Now, we understand it wasn't as quiet as it seemed. We're at the beginning stages of being able to really understand the formation of the Milky Way."

Necib and her colleagues combined FIRE and Gaia data and analyzed them by applying deep learning methods, like algorithms and artificial neural networks. "We can't stare at seven million stars and figure out what they're doing," Necib said.

A deep learning model was trained to track each star in the galaxies simulated by FIRE and then label them as native to the host galaxy, or the product of galactic mergers.

Bryan Ostdiek, a postdoctoral research scholar at Harvard University, formerly worked on the Large Hadron Collider, the world's largest and most powerful particle accelerator, near Geneva. He brought over a lot of the machine learning techniques from that project to help interpret Gaia data.

When they applied their techniques to Gaia data, the neural network ranked and labeled the stars. The researchers tested its accuracy by searching for stars that are known to be the product of mergers with dwarf galaxies in the past, and the model correctly identified them.

Then, the model identified a cluster of 250 stars, or what would become known as Nyx.

"Your first instinct is that you have a bug," Necib said. "And you're like, 'Oh no!' So, I didn't tell any of my collaborators for three weeks. Then I started realizing it's not a bug, it's actually real and it's new."

She checked through past research and this star stream had never been identified before, so Necib had the privilege of naming it.

"This particular structure is very interesting because it would have been very difficult to see without machine learning," she said. "I think we reached a point in astronomy where we are not data limited anymore. This project is an example of something that would have not been possible a few years ago, the culmination of developments in data with Gaia, high resolution simulations, and machine learning methods."

Necib and her colleagues will investigate Nyx more in the future using ground-based telescopes, including the W.M. Keck Observatory in Hawaii and the Giant Magellan Telescope in Chile. This will enable them to study the chemistry of the stars, and eventually discern their ages and the time scale over which they merged with the Milky Way.

"The stars that are born outside the Milky Way have different chemicals than the ones that are born here," Necib said.

Gaia will release more data in 2021 with new information about the already 100 million stars in its catalog. And this new data could shed more light on Nyx.

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