The Milky Way Likely "Ate" Another Unidentified Galaxy In The Distant Past

A visualization, which shows the Jhelum stream (in orange), and some other streams around the Milky Way. Credit: A. Price-Whelan, CC BY 4.0

The Milky Way has been known to be snacking on its neighbors for billions of years, slowly but surely merging with several of the many dwarf galaxies that surround our galaxy. The leftovers of such interactions can be seen today in the form of stellar streams wrapping around the galaxy like ribbons.

A newly discovered "ribbon" is the Jhelum stellar stream. Due to its location in the sky, astronomers considered this stream to be part of a collision between the Milky Way and the Gaia-Enceladus-Sausage dwarf galaxy, thought to have occurred between eight and 11 billion years ago.

Now new observations have revealed that Jhelum doesn’t come from the Sausage dwarf galaxy after all. Astronomers were able to study the light spectrum of stars in the Jhelum stream, which gave them an idea of what the stars are made of. Stars that formed together will have a similar composition. This was tracked using the Apache Point Observatory Galactic Evolution Explorer (APOGEE) survey, part of the Sloan Digital Sky Survey (SDSS).

The composition allows selecting the stars that formed and belong together. By combining this with detailed data about their position and motion from the European Space Agency’s Gaia, the team showed that the Jhelum stream did not match the other evidence of the Sausage collision. Its origin was elsewhere.

"Like fingerprints or tags, the chemical properties of stars in a stream can be used to tell them apart from other streams -- but more than that, having the chemical makeup, positions and motions together is incredibly valuable, and just goes to show the benefit of combining APOGEE and Gaia," senior author Professor Allyson Sheffield, from LaGuardia Community College, said in a statement.

The Jhelum stream is only observable from the southern hemisphere so it was thanks to the southern APOGEE instrument (APOGEE-2) coming online that the team was able to use to identify the right stars and then work out who were the actual members of the stream.

"Measuring the tracks, or orbits of stars in the stream allows us to almost turn back the cosmic clock, and reveal where the stream itself may have come from," added co-author Aidan Subrahimovic, an astrophysics student at City University of New York.

Researchers can’t link the Jhelum stream to any specific merger with a dwarf galaxy or a globular cluster and so its origins currently remain unknown. However, future work might help researchers find an answer to the origin of this and other stellar streams in our Milky Way.

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