A team of astronomers from Israel and the United States has discovered a vast stream of 250 stars of extragalactic origin in the vicinity of our Solar System.
Stellar streams arcing high over the Milky Way Galaxy are remnants of accreted dwarf galaxies and star clusters. Image credit: NASA / JPL-Caltech / R. Hurt, SSC & Caltech.
Galaxies form by swallowing other galaxies, said lead author Dr. Lina Necib, a postdoctoral researcher in the Walter Burke Institute for Theoretical Physics at Caltech.
Weve 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, with access to a lot of smaller structures, we understand it wasnt as quiet as it seemed.
Its very powerful to have all these tools, data and simulations. All of them have to be used at once to disentangle this problem.
Were at the beginning stages of being able to really understand the formation of the Milky Way.
Dr. Necib and colleagues developed a method of tracking the movements of stars in virtual galaxies and labeling the stars as either born in the host galaxy or accreted as the products of galaxy mergers.
The two types of stars have different signatures, though the differences are often subtle.
These labels were used to train the deep learning model, which was then tested on simulations from the FIRE (Feedback In Realistic Environments) project.
After they built their catalogue of accreted stars, the astronomers applied it to the second data release from ESAs star-mapping satellite Gaia.
We asked the neural network, Based on what youve learned, can you label if the stars were accreted or not? Dr. Necib said.
The researchers first checked to see if it could identify known features of the Milky Way Galaxy.
These include the Gaia sausage, the remains of a dwarf galaxy that merged with the Milky Way about 6-10 billion years ago and that has a distinctive sausage-like orbital shape.
It has a very specific signature. If the neural network worked the way its supposed to, we should see this huge structure that we already know is there, Dr. Necib said.
The Gaia sausage was there, as was the stellar halo, and the Helmi stream, another known dwarf galaxy that merged with the Milky Way in the distant past and was discovered in 1999.
The model identified another structure in the analysis: a cluster of 250 stars, rotating with the Milky Ways disk, but also going toward the Galactic center.
Your first instinct is that you have a bug, Dr. Necib said.
And youre like, Oh no! So, I didnt tell any of my collaborators for three weeks. Then I started realizing its not a bug, its actually real and its new.
The authors argue that the newfound stellar stream, named Nyx, is a remnant of a disrupted dwarf galaxy that merged with the Milky Way billions years ago.
The Nyx stream provides the first indication that such an event occurred in the Milky Way, they said.
Such stellar streams are thought to be globular clusters or dwarf galaxies that have been stretched out along its orbit by tidal forces before being completely disrupted.
The scientists identified an additional stream, dubbed Nyx-2, with similar motion and stellar characteristics, but opposite velocity.
This suggests that Nyx-2 is related to Nyx and might actually be debris from a separate passage of the same satellite galaxy, they said.
The discovery is reported in the journal Nature Astronomy.
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L. Necib et al. Evidence for a vast prograde stellar stream in the solar vicinity. Nat Astron, published online July 6, 2020; doi: 10.1038/s41550-020-1131-2
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