This new star cluster has been named Nyx, after 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 viewed from the side.
The 250-star group rotates with the Milky Way’s galactic disk, where most of the stars in the galaxy are located. But Nyx stars are also moving towards the center of the galaxy.
“The two possible explanations here are that they are the remains of a [galactic] fusion, or that they are disk stars that jolted in their new orbits due to a collision with the Milky Way disk, “said Lina Necib, study author and postdoctoral fellow in theoretical physics at the California Institute of Technology, in an email to CNN.
Mergers are how galaxies grow. They swallow smaller neighboring dwarf galaxies. Nyx was probably once a dwarf galaxy that merged with the Milky Way.
“If Nyx is indeed a dwarf galaxy, it would have collided with the Milky Way disk at a low level [angle]”Necib said.” What makes Nyx special is its ‘prograde’ orbit, the fact that [the stars] are co-rotating with the disk. We have not detected this type of fusion before. “
A new way to study galaxies
Necib and his 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 motion of stars and dark matter in the Milky Way, which can tell us a lot about our galaxy.
“If there are clusters of stars that move together in a particular way, that generally tells us that there is a reason why they move together,” he said.
Since Gaia’s launch, the space observatory has been mapping a billion stars in our galaxy and beyond, aiming to create an incredibly accurate 3D map by the end of the mission in 2022.
And Caltech is just one of many institutions where researchers are dedicated to creating detailed simulations of galaxies that include all the current information on galactic formation and evolution through the FIRE project, or Feedback in realistic settings. This project allows researchers to simulate the formation and evolution of galaxies over time.
“Galaxies are formed by swallowing other galaxies,” said Necib. “We assumed that the Milky Way had a silent merger history, and for a time it was about how quiet it was because our simulations show a lot of mergers. Now, we understand that it was not as quiet as it seemed. We are in the early stages of being able to really understand the formation of the Milky Way. “
Necib and his colleagues combined data from FIRE and Gaia and analyzed it using deep learning methods, such as algorithms and artificial neural networks. “We cannot look at seven million stars and find out what they are doing,” said Necib.
A deep learning model was trained to track each star in the FIRE-simulated galaxies and then tag them as native to the host galaxy, or the product of galactic mergers.
Bryan Ostdiek, a postdoctoral researcher at Harvard University, previously worked at the Large Hadron Collider, the world’s largest and most powerful particle accelerator, near Geneva. He brought many of the machine learning techniques from that project to help interpret Gaia’s data.
When they applied their Gaia data techniques, the neural network classified and labeled the stars. The researchers tested their accuracy by looking for stars known to have been the product of mergers with dwarf galaxies in the past, and the model correctly identified them.
Then the model identified a group of 250 stars, or what would become known as Nyx.
“Your first instinct is that you have a mistake,” said Necib. “And you think, ‘Oh no!’ So, I didn’t tell any of my collaborators for three weeks. Then I started to realize that it’s not a mistake, it’s actually real and it’s new. “
He reviewed past research and this stellar 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,” he said. “I think we reached a point in astronomy where we no longer have limited data. This project is an example of something that would not have been possible a few years ago, the culmination of developments in data with Gaia, high-resolution simulations, and machine learning methods. “
Necib and colleagues will investigate Nyx further in the future using ground-based telescopes, including the WM Keck Observatory in Hawaii and the Giant Magellan Telescope in Chile. This will allow them to study the chemistry of the stars and, finally, to discern their ages and the time scale in which they merged with the Milky Way.
“Stars that are born outside the Milky Way have different chemicals than those that are born here,” Necib said.
Gaia will release more data in 2021 with new information on the already 100 million stars in its catalog. And this new information could shed more light on Nyx.
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