The Milky Way’s most recent significant instance of galactic cannibalism occurred quite recently.

Skin stretches and gets more wrinkled with age, but the galaxy is a type of Benjamin Button—as our paper demonstrates, it has fewer wrinkles growing older. ”
New images produced by the recently launched Gaia space telescope confirm that our galaxy has recently ingested a small galaxy. Even more importantly, manned exploration has taught scientists and historians that the last major collision with another galaxy may have taken place billions of years later than everyone thought.

The Milky Way has previously been recognized to have accumulated and increased in size by several jolting collisions; these unbearably forceful merging events cause holder galaxies to be torn apart by the Milky Way’s powerful gravity pulling force while it is coalescing into a spiral home. These collisions disseminate stars from the devoured galaxy around a shell formed around the main disk of the Milky Way, recognizable by its individual spiral segments. These episodes of galactic cannibalism also create ‘ripples’ such as ‘wrinkles’ in the Milky Way that impact on different ‘family’ of stars with different ‘birth right’ at different ‘mileage’ at different scales.

As a positioning tool with a position and motion measurement of more than 100000 stars in the local solar neighborhood in a full census of stellar objects in monitors, Gaia set out to do for the Milky Way what a wrinkle counter does for a face: tell the age of the Galaxy.

This is ironic, and it is also stated in a different context: ‘As you get wrinklier with age, our studies suggest that the Milky Way galaxy is the opposite: it gets less wrinkled or more smooth with time. ’Going back in time by observing how these wrinkles fade, researchers can determine when the Milky Way had the last major collision – and shocking, this event occurred billions of years later than it had been hypothesized.

These galactic wrinkles were only discovered by Gaia in the year 2018; this is the first time when these have been studied intensively to show the timing of the collision that led to their formation.

Halo stars moving in strange ways

The outer halo of the Milky Way contains stars moving with rather exotic orbits, with many of them thought to be the remnants of galaxies that the Milky Way has consumed.

Most of them are thought to be debris from the so-called “last major merger” which is the last big merger that the Milky Way had with other galaxies. Researchers believe that the last major merger involved a giant dwarf galaxy and the event is called the Gaia-Sausage-Enceladus (GSE). It is believed to had seeded the Milky Way with stars on orbits that take them inside the Galactic Center. It is estimated that the GSE event occurred between 8 and 11 billion years ago when the Milky Way galaxy was just forming.

Since 2020, Thomas and his team have been matching the folds in the Milky Way to simulations of how galactic collisions and mergers might have formed them. However, the Gaia observations of these oddly moving stars — made available in the third Gaia data release in 2022 — suggest that these odd stellar bodies may have been dumped by a different merger.

These simulated mergers allow us to see how the shapes and numbers of the wrinkles changes with time — this helps us to find out the correct time when the simulation matches the Gaia data of the Milky Way today — the method we also used in this new study, Donlon said. In this way, we realized that the wrinkles were caused by a collision with a small galaxy that happened with the Milky Way 2,7 billion years ago, so we called this event Virgo Radial Merger.

“If the wrinkles of stars are to be as clean as are observed by Gaia, then they must have merged with us no earlier than 3 billion years ago and not before 5 billion years later than generally believed. ” Other members include Heidi Jo Newberg of Rensselaer Polytechnic Institute, added. ‘New wrinkles of stars are produced each time again the stars move to and fro direction through the Milky Way’s galaxy hub If they had been here eight million years ago the wrinkles would lie one besides another so closely that we couldn’t perceive them anymore as individual entities’.

The recent study of the records of Gaia has raised the suspicion that a huge merger in the ancient history of the Milky Way is not necessarily required to account for some of the peculiar motions of stars in the galactic. It also, of course, calls into question all of the previously identified stars aligned with GSE merger as well.

“And we find that a large fraction of the stars in the Milky Way, that is, most of the thick disk and halo, have only formed within the last few billion years — a stark contrast from previous opinion,” Donlon said. Most of the existing views and ideas of how the galaxy including the Milky Way expands, would rather anticipate that a head-on clash with this size of dwarf galaxy in the recent past would occur extremely seldom.

The team also believes that they call the ‘Virgo Radial Merger’ happened to our galaxy a family of other small dwarf galaxies and star clusters and all of them would have been consumed by Milky Way at the same time.

Additional study and data from that satellite could demonstrate that none or some of the objects thought to be involved in the GSE event are actually part of the more recent Virgo Radial Merger.

This new research is the latest one among the rich set of discoveries made based on Gaia data that is reshaping the history of the Milky Way.

Such cosmic revisionism has been made possible because Gaia is capable of observing a large number of stars over the Earth and it has produced the most intricate map of the positions, distances and motions of about 1. 5 billion stars up to the current time.

“The history of the Milky Way is continuously being revised, especially with 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 in the journal Monthly Notices of the Royal Astronomical Society.

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