New findings from the Gaia space telescope suggest that, cosmically speaking, the Milky Way may have destroyed a small galaxy a long time ago. In fact, it looks like the last major collision between our galaxy and another has occurred billions years later than previously suspected.
It has long been known that the Milky Way grew through a series of violent collisions in which smaller galaxies were torn apart by the immense gravitational pull of our solar system’s spiral house. These collisions disperse the stars in the engulfed galaxy across the halo surrounding the Milky Way’s main disk and its distinctive spiral arms. These bouts of galactic cannibalism also send out “wrinkles” across the Milky Way that affect different “families” of stars of different origins in different ways.
Gaia aims to retell the history of the Milky Way by counting its wrinkles, with the ability to pinpoint the position and motion of more than 100,000 local stars in the solar system across the full catalog of stellar objects on monitors.
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“We get more wrinkled as we get older, but our studies reveal that the opposite is true for the Milky Way. It’s a kind of cosmic Benjamin Button, getting less wrinkled over time,” said Thomas Donlon, study team leader at Rensselaer Polytechnic Institute and University. The scientist from Alabama said the following in his statement. “By looking at how these wrinkles disperse over time, we can track when the Milky Way experienced its last major collision, and it turns out it happened billions of years later than we thought.”
These galactic wrinkles were only discovered by Gaia in 2018; This marks the first time a comprehensive investigation has been conducted to uncover the timing of the collision that created them.
Halo stars move in strange ways
The halo of our galaxy is full of stars with strange orbits, and many of them are believed to be the “leftovers” of galaxies once swallowed by the Milky Way.
Many of these stars are believed to be debris from what has been called the “last great merger,” a reference to the last time the Milky Way had a significant collision with another galaxy. Scientists think this last 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 populate the Milky Way with stars in orbits that bring them closer to the Galactic Center. The GSE event is thought to have occurred between eight and 11 billion years ago, when the Milky Way was in its infancy.
Since 2020, Thomas and his team have been comparing the Milky Way’s wrinkles with simulations of how galactic collisions and mergers might have created them. However, Gaia observations of these strange orbiting stars, published as part of the space telescope’s Data Release 3 in 2022, suggest that these strange stellar objects may have been deposited by a different merger event.
“Using these simulated mergers, we can see how the shapes and numbers of wrinkles change over time. This allows us to pinpoint when the simulation best matches what we see in the real Gaia data of the Milky Way today – a method we use in this regard.” There’s also a new study,” Donlon explained. “By doing this, we found that the wrinkles probably originated from a dwarf galaxy that collided with the Milky Way about 2.7 billion years ago. “We called this event the Virgo Radial Convergence.”
“For the stars’ wrinkles to be as clear as they appear in the Gaia data, they would have to have joined us less than three billion years ago, at least five billion years later than previously thought,” said team member Heidi Jo Newberg, also of Rensselaer. Polytechnic Institute said. “As stars rock back and forth at the center of the Milky Way, new stellar wrinkles form. If they had joined us eight billion years ago, there would be so many wrinkles side by side that we would no longer be able to see them.” “We see these as separate features.”
Recent examination of Gaia’s observations raises the question of whether a massive ancient merger in the early history of the Milky Way is really necessary to explain the strange orbits of some stars in the galactic. It also casts doubt on any stars previously associated with the GSE merger.
“This conclusion that most of the Milky Way has only joined us in the last few billion years is a big change from what astronomers had previously thought,” Donlon said. “Many popular models and ideas about how the Milky Way grew expect a head-on collision with a dwarf galaxy of this mass in the near future to be very rare.”
The team also thinks that the Virgo Radial Merger introduced a family of other small dwarf galaxies and star clusters into our galaxy, all of which may have been swallowed by the Milky Way around the same time.
Future studies and data from Gaia may indicate whether any objects previously associated with the GSE event are actually associated with the more recent Virgo Radial Merger.
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This new research is the latest to emerge from Gaia data that is rewriting the history of the Milky Way.
Such cosmic revisionism was made possible by Gaia’s unique ability to explore large numbers of stars on Earth; This has allowed the space telescope to compile an unrivaled map of the positions, distances and movements of nearly 1.5 billion stars so far.
“The history of the Milky Way is constantly being rewritten in no small part by new data from Gaia,” Donlon concluded. “Our picture of the Milky Way’s past has changed dramatically from even a decade ago, and I think our understanding of these mergers will continue to change rapidly.”
The team’s research was published in May. Monthly Notices of the Royal Astronomical Society.