New findings from the Gaia Space Telescope indicate that the Milky Way cosmically cannibalized a small galaxy not so long ago. In fact, the last major collision between our galaxy and another galaxy appears to have occurred billions years later than previously suspected.
It has long been believed that the Milky Way grew through a series of violent collisions, with smaller galaxies torn apart by the immense gravitational influence of our solar system's spiral home. These collisions distribute stars from the devoured galaxy across the halo surrounding the Milky Way's main disk and its distinctive spiral arms. These bouts of galactic cannibalism also cause 'ripples' to flow through the Milky Way, affecting different 'families' of stars, with different origins, in different ways.
With its ability to accurately determine the position and motion of more than 100,000 stars in the solar system within the entire catalog of stellar bodies in monitors, Gaia aims to retell the history of the Milky Way by counting its ripples.
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"We become more wrinkled as we age, but our research shows that the opposite is true for the Milky Way. It's a kind of cosmic Benjamin Button, becoming less wrinkled over time," said Thomas Donlon, research team leader at the Rensselaer Polytechnic Institute and University. from the Alabama scientist, said in a statement. "By watching these ripples disappear over time, we can pinpoint when the Milky Way experienced its last major crash - and it turns out this happened billions of years later than we thought."
These galactic ripples were only discovered by Gaia in 2018; this is the first time extensive research has been done to reveal the timing of the collision that created them.
Halo stars move in strange ways
Our Milky Way's halo is populated with stars that have strange orbits, many of which are believed to be the 'remnants' of galaxies that the Milky Way once devoured.
Many of these stars are believed to be the wreckage of the so-called "last great merger," referring to the last time the Milky Way experienced a significant collision with another galaxy. Scientists believe this latest major collision may have involved a massive dwarf galaxy, and the event is known as the Gaia-Sausage-Enceladus (GSE) merger. It is thought to have provided the Milky Way with stars in orbits that bring them close to the galactic center. The GSE event is believed to have occurred between eight and eleven billion years ago, when the Milky Way was still in its infancy.
Since 2020, Thomas and his team have been comparing the Milky Way's ripples with simulations of how galactic collisions and mergers might have caused them. However, the Gaia observations of these strangely spinning stars - released as part of the space telescope's Data Release 3 in 2022 - indicate that these strange stellar bodies could have been deposited by another merger event.
'Using these simulated mergers, we can see how the shapes and number of wrinkles change over time. This allows us to determine the exact time when the simulation best matches what we see today in real Gaia data from the Milky Way - a method we used here. also a new study," Donlon explains. "By doing this, we found that the ripples were likely caused by a dwarf galaxy that collided with the Milky Way about 2.7 billion years ago. We called this event the Virgo Radial Merger."
"For star ripples to be as bright as they appear in Gaia data, they must have come to us less than three billion years ago - at least five billion years later than previously thought," said team member Heidi Jo Newberg, also van Rensselaer. Polytechnic Institute, said. 'Every time the stars swing back and forth through the center of the Milky Way, new stellar ripples form. If they had joined us eight billion years ago, there would have been so many ripples next to each other that we would no longer see them. them as separate features."
The recent investigation of Gaia's observations raises the question of whether a massive ancient merger in the Milky Way's early history is really necessary to explain the strange orbits of some stars in the galactic system. It also casts doubt on all the stars previously associated with the GSE merger.
"This result - that much of the Milky Way has only come to us in the past few billion years - is a big change from what astronomers thought until now," Donlon said. "Many popular models and ideas about how the Milky Way grows expect that a recent head-on collision with a dwarf galaxy of this mass will be very rare."
The team also thinks that the Virgo Radial Merger brought a family of other small dwarf galaxies and star clusters to our Milky Way, all of which would also have been devoured by the Milky Way around the same time.
Future research and data from Gaia could reveal whether objects previously associated with the GSE event are actually linked to the more recent Virgo Radial Merger.
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This new research is the latest in a wealth of results coming from Gaia data that are rewriting the history of the Milky Way.
Such cosmic revisionism has been made possible thanks to Gaia's unique ability to survey a large number of stars above Earth, allowing the space telescope to compile an unprecedented map of the positions, distances and motions of about 1.5 billion stars so far.
"The history of the Milky Way is constantly being rewritten, thanks in no small part to new data from Gaia," Donlon concluded. "Our view of the Milky Way's past has changed dramatically from a decade ago, and I think our understanding of these mergers will continue to change rapidly."
The team's research was published in Maythe journal Monthly Notices of the Royal Astronomical Society.
