The chaotic process of multiple collisions and mergers with other galaxies, which occurs in many galaxies throughout the Universe, gave rise to the vast structure of the Milky Way.
The Large and Small Magellanic Clouds are the two satellite galaxies that the Milky Way is gradually absorbing. Researchers have long accepted that the last significant merger occurred between 8 and 11 billion years ago. New evidence, however, points to a far more recent date of less than 3 billion years ago.
The European Space Agency’s (ESA) Gaia project is responsible for this newfound understanding of our galaxy’s history. Gaia was launched in 2013 with the goal of mapping one billion celestial objects, mostly stars, and precisely determining their locations and motions.
These findings are presented in a recent study, “The Debris of the ‘Last Major Merger’ is Dynamically Young,” published in the Monthly Notices of the Royal Astronomical Society. The primary author, Thomas Donlon, has researched Milky Way mergers for several years. He is a post-doctoral researcher in Physics and Astronomy at the University of Alabama, Huntsville.
The Milky Way is left with unique “wrinkles” from each merger; these terms encompass a variety of morphologies, such as phase space folds, caustics, chevrons, and shells. These wrinkles impact the stars’ motion inside the galaxy. Gaia detects the remains of the last massive merger by precisely monitoring the positions and velocities of stars.
“We get wrinklier as we age, but our work reveals that the opposite is true for the Milky Way. It’s a sort of cosmic Benjamin Button, getting less wrinkly over time,” said Donlon in a press release. “By looking at how these wrinkles dissipate over time, we can trace when the Milky Way experienced its last big crash – and it turns out this happened billions of years later than we thought.”
The evidence of the Milky Way’s last major merger includes an iron-rich (Fe/H) region where stars have highly eccentric orbits, indicative of a common origin. This group of stars, known as ‘the Splash,’ may have originated from a progenitor galaxy. Their odd orbits were likely altered during the merger.
Two main theories explain the merger evidence: one suggests a progenitor dwarf galaxy named Gaia Sausage/Enceladus (GSE) collided with the Milky Way’s proto-disk between 8 and 11 billion years ago. The other theory attributes the evidence to the Virgo Radial Merger (VRM), which occurred less than 3 billion years ago.
“These two scenarios make different predictions about observable structure in local phase space because the morphology of debris depends on how long it has had to phase mix,” the authors explain.
The wrinkles were first identified in Gaia data in 2018. “We have observed shapes with different morphologies, such as a spiral similar to a snail’s shell. The existence of these substructures has been observed for the first time thanks to the unprecedented precision of the data brought by Gaia satellite,” said Teresa Antoja, the study’s first author, in 2018.
Subsequent Gaia data supports the more recent VRM scenario. “For the wrinkles of stars to be as clear as they appear in Gaia data, they must have joined us less than 3 billion years ago – at least 5 billion years later than was previously thought,” said co-author Heidi Jo Newberg from the Rensselaer Polytechnic Institute.
“If they’d joined us 8 billion years ago, there would be so many wrinkles right next to each other that we would no longer see them as separate features,” Newberg added.
There is evidence supporting the ancient GSE merger, even though the case for the more recent VRM merger is stronger. While certain stars that allude to the earlier merger might still be connected to the GSE, others might be a component of the VRM.
To comprehend these mergers, it is essential to use simulations. Researchers run several simulations to determine which best fits the Gaia data. “Our goal is to determine the time that has passed since the progenitor of the local phase-space folds collided with the Milky Way disc,” the authors write.
“By doing this, we found that the wrinkles were likely caused by a dwarf galaxy colliding with the Milky Way around 2.7 billion years ago. We named this event the Virgo Radial Merger,” said Donlon.
With each new piece of data that Gaia offers, scientists are putting together a more complete picture of the complex history of the Milky Way. “The Milky Way’s history is constantly being rewritten at the moment, in no small part thanks to new data from Gaia,” adds Donlon. “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.”
“This finding improves what we know of the many complicated events that shaped the Milky Way, helping us better understand how galaxies are formed and shaped – our home galaxy in particular,” said Timo Prusti, Project Scientist for Gaia at ESA.
