Researchers have uncovered relics of old galaxies colliding with each other to form the Milky Way, shedding new light on the evolution of humankind's home galaxy.
Smaller Galaxies And One Big 'Blob' Of Stars
A team of astronomers from the University of Groningen in the Netherlands have identified numerous galactic events that have led to the formation of the Milky Way.
The team, headed by PhD researcher Helmer Koppelman, have located five small star groups that merged with dwarf galaxies and one big "blob" of stars that they say is the vestige of a larger merger event.
The study, which is published in the Astrophysical Journal Letters, was based on information obtained from the second release of data from the European Space Agency's Gaia mission, which took place between July 2014 and May 2016. Gaia's objective was to examine the position and movement of more than 1.7 billion stars in the Milky Way galaxy.
Did Small Or Large Galaxies Merge To Form The Milky Way?
Koppelman, who is part of a research group under Amina Helmi, professor of galaxy evolution at the Kapteyn Astronomical Institute of the University of Groningen, has the goal of studying the evolution of the Milky Way.
Prior to the study, astronomers have always believed that smaller galaxies merged to form larger galaxies. However, when it comes to the Milky Way, the question remains whether it was formed by the collision of dwarf galaxies or larger galaxies.
Koppelman set out to study Gaia Data Release 2 as soon as it was made available. He specifically focused on data on stars situated in the galactic halo. This is the sphere-shaped cloud of stars surrounding the disc of the Milky Way. The halo is believed to be the remains of early galactic mergers that form larger galaxies.
More Details On Halo Stars
The study centered on stars 3,000 light-years from the sun, since stars closer to the solar system have more accurate position and movement measurements. To pinpoint these specific stars, Koppelman first had to filter out the stars at the disc of the Milky Way. He ended up with around 6,000 halo stars that moved around the galactic disc.
By determining the trajectory of these stars, he was able to identify the stars that had a similar origin. Koppelman found five small clusters of stars that are the remains of five galactic mergers.
He also discovered a huge "blob" of stars that move in a retrograde motion against the galactic disc, suggesting that the stars were once part of a larger galaxy that merged into the Milky Way. Koppleman says the larger merger could also have remodeled the galactic disc.
"At this point in time, we can say that our Milky Way was shaped by a massive merger event and some smaller mergers," Koppelman says.
Looking Into The Helmi Stream
The study also looked into the Helmi Stream, a dwarf galaxy streaming into the Milky Way that was discovered by Koppelman's mentor in 1999.
Prior to second data release from Gaia, there were less than 20 stars known to be part of the Helmi Stream. However, Koppelman was able to identify hundreds of stars that are part of the star group.
Koppelman and his team plan to examine the data on stars that are more than 3,000 light-years away. Further study paired with observations of galactic evolution models should help provide more insight into the beginning and growth of the Milky Way.