Two of the oldest building blocks of the Milky Way have just been found

Galaxies grow by taking material from intergalactic space, but also by swallowing other galaxies. We see that happening in the universe and we know that it must have happened in the past with our own galaxy, the Milky Way. Thanks to the Gaia Observatory, astronomers have now found two of the oldest mergers to have occurred in our Milky Way.

Gaia is a European Space Agency mission responsible for the most accurate map of the Milky Way ever created. Such a map tells us where billions of stars are today, but can also be used for galactic archaeology: finding out where some stars come from.

A galaxy merger is a slow and messy affair. Usually much more gas is added, eventually creating new stars. Older stars from the original galaxies are mixing together, and if you just photographed them you wouldn’t be able to tell them apart. But Gaia data, combined with the spectra of stars from the Sloan Digital Sky Survey (DR17), have allowed us to reveal much more.

Gaia’s huge map also provides information about the movement of stars. If a group of stars all move in the same way, it’s an indication that they may be related, but the proof comes from their chemical composition.

Stars are mainly composed of hydrogen and helium, but they also contain some other elements. This is known as metallicity and can act like a fingerprint and date stamp. The older the star, the lower the metallicity. Stars created the elements beyond helium, so older stars had fewer metals to play with as they formed. Also, stars formed in the same area will have a similar composition.

By bringing metallicity and motion together, researchers can find out whether these groups belong together.

They have done this several times, with stellar streams such as the Pontus Stream and the ‘poor old heart’ of the Milky Way in 2022, as well as the most recent major merger, caused by the collision with the Gaia Enceladus/Worst galaxy between 8 and 10 billion years ago . The two new components are called “Shakti” and “Shiva” and are much older. They merged with the proto-Milky Way between 12 and 13 billion years ago.

“What’s really amazing is that we can detect these ancient structures at all,” Khyati Malhan of the Max Planck Institute for Astronomy (MPIA), who led the research, said in an emailed statement to IFLScience. “The Milky Way has changed so significantly since the birth of these stars that we wouldn’t expect to recognize them so clearly as a group – but the unprecedented data we’re getting from Gaia has made this possible.”

“When we imaged the orbits of all these stars, two new structures stood out among the stars with a certain chemical composition,” Khyati said. “We called them Shakti and Shiva.”

The exciting difference between Shakti and Shiva compared to the Ancient Heart of the Galaxy is that these components are spread far from the core of our Galaxy. Shakti spins further away than Shiva and in a more circular manner. If we turned back time 12 billion years, we wouldn’t see a galaxy with spiral arms in a thin disk. We would see the messy streams of stars from multiple collisions.

“Shakti and Shiva could be the first two additions to the ‘poor old heart’ of our Milky Way, triggering its growth into a large galaxy,” says co-author Hans-Walter Rix, also from MPIA and the lead “galactic archaeologist” of the 2022 work, said in another statement.

Future Gaia releases could reveal more streams and more ancient components of the Milky Way. They could provide important missing steps in the formation and evolution of our own galaxy.

The research has been published in The Astrophysical Journal.

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