A "vast" stream of stars that appear to have come from outside the Milky Way has been discovered moving toward the galaxy center. Researchers believe the stream, which contains around 200 stars, is the result of a merger event between the Milky Way and a dwarf galaxy, with the latter being consumed by our home galaxy a long time ago.
Lina Necib, a Postdoctoral Fellow at the California Institute of Technology, and colleagues were attempting to build a catalog of stars in the Milky Way that were born outside the galaxy. They created a machine learning program that can distinguish between stars from the Milky Way and those from outside it. They used this to look at data from Gaia, a mission to categorize one billion stars in the Milky Way.
Necib had initially been interested in building a link between dark matter, the mystery form of matter that makes up most of the universe, and stars. Scientists had previously found evidence to suggest the kinematics—which describes motion without reference to the force causing it—of dark matter were correlated with accreted stars. These are stars born in other galaxies and brought in through mergers.
When looking at the catalog, Necib noticed a huge stream of stars that had never been noticed before. "I was not expecting to see new stellar streams, but it was a great surprise," she told Newsweek. "Using the catalog, I started looking at kinematic clusters, basically stars that have similar velocities. I found evidence for known structures, such as the Gaia Sausage, and the Helmi stream, but I also found this new cluster of stars that I did not know about." She named the stream Nyx, after the Greek Goddess of night.
The stream of stars was found to be rotating with the Milky Way, but also moving toward the center. Necib and colleagues have published their findings in the journal Nature Astronomy.
In the paper, the team says this is the "first indication" that a dwarf galaxy merged with our own and its stars were dragged into the Milky Way disk plane. While the team cannot be sure the newly discovered stream of stars is from a galaxy merger, they say their results "strongly favor" this interpretation.
"It could also be stars from the disk of the Milky Way that are vibrating because of a collision of the disk," Necib said. "To figure this out, I'm collaborating with observers to study the chemical abundances of the Nyx stars, so we can narrow down its origin."
She said if it is evidence of a merger between the Milky Way and a dwarf galaxy, Nyx would have a dark matter component. "This means that there might be a fraction of dark matter in the solar neighborhood with interesting kinematics, that might affect our direct detection experiments."
The next step in the research is to understand the origin of these stars, she said. "Using the next Gaia data release, we will also look at the extension of the Nyx stars further from the plane of the Milky Way and build a coherent story of the formation of the Milky Way."
