ESA’s Gaia Satellite Finds Proof Of Ancient Galactic Mergers In The Halo Of The Milky Way

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It seems that the Milky Way was shaped into existence by a “massive merger event” with another large galaxy, as well as several smaller mergers, astronomers from the University of Groningen in the Netherlands have uncovered.

According to a study published yesterday in The Astrophysical Journal Letters, these ancient galactic mergers have left remnants inside the Milky Way’s halo, which have been detected by the Dutch researchers analyzing data from the second slew of Gaia recordings released in April.

As the Inquisitr previously reported, the European Space Agency (ESA) has recently made public the second batch of data from the Gaia satellite, which includes a vast star map comprising of 1.2 billion stars located as far as 8,000 light-years from Earth.

Astronomers have long wanted to know whether our galaxy was formed through one massive galactic merger or a series of smaller collisions with other galaxies. To get to the bottom of this big question, the Dutch researchers turned their attention to the Milky Way’s halo — the outer region of the galaxy which surrounds the main disk and galactic bulge in a spherical cloud of stars.

“Our aim is to study how the Milky Way has evolved,” says study lead author Helmer Koppelman, a Ph.D. student who has been working on this research project ever since the Gaia data was released together with Prof. Amina Helmi, one of the original participants in ESA’s Gaia mission.


Since the best place to look for ancient remnants of our galaxy’s beginnings is the galactic halo, the team combed through the Gaia data and identified close to 6,000 stars residing in the Milky Way’s halo.

“We collected information from stars within 3,000 light-years of the sun, as the accuracy of the position and movement is highest for stars that are near us,” reveals Koppelman.

These stars were carefully filtered from those belonging to the Milky Way disk, which are easy to spot because they gravitate around the disk’s center, explains the astronomer.

The team’s efforts led to two major discoveries that are now helping scientists better understand how our galaxy evolved, reports the International Business Times.


For one thing, the researchers detected five small clusters of stars that, according to their trajectory, share the same origin and seem to be dating back to small-scale merger events.

“We discovered five small clusters which we believe are remnants of five merger events,” says Koppelman.

In addition, his team also came across a massive chunk of hundreds of stars that are related to each other and share the same history. These stars belong to the “Helmi stream” — a group of almost 20 stars, now vastly expanded, identified by Koppelman’s supervisor in 1999 and which have been established as relics of the same ancient merger event.

Because the stars in the “Helmi stream” have a retrograde movement compared to the disk of the Milky Way, they appear to be “the result of a merger with a large galaxy,” Koppelman points out.

“In fact, we believe that this merger event must have remodeled the disk in our Milky Way,” he says.

“At this point in time, we can say that our Milky Way was shaped by a massive merger event and some smaller mergers.”