The Milky Way is only as massive as it is because of collisions and mergers with other galaxies. This is a messy process and we see the same thing happening to other galaxies throughout the universe.
Currently, we see the Milky Way nibbling on its two satellite galaxies, the Large and Small Magellanic Clouds. Their fate is likely sealed and they will be absorbed into our galaxy.
Scientists believed that the last major merger occurred in the distant past of the Milky Way, between 8 and 11 billion years ago. But new research reinforces the idea that it was much more recent: less than 3 billion years ago.
This new insight into our galactic history comes from ESA’s Gaia mission. Launched in 2013, Gaia is diligently mapping 1 billion astronomical objects, mostly stars. It measures them repeatedly, ensuring accurate measurements of their positions and movements.
A new article published in Monthly Notices of the Royal Astronomical Society presents findings. It’s called “The ‘Last Great Fusion’ Debris Is Dynamically Young.” The lead author is Thomas Donlon, a postdoctoral researcher in physics and astronomy at the University of Alabama, Huntsville. Donlon studied mergers in the Milky Way for several years and published other papers on the subject.
Every time another galaxy collides and merges with the Milky Way, it leaves a wrinkle. “Wrinkles” is clearly not a scientific term. It is an umbrella term for several types of morphologies, including phase space folds, caustics, darts, and shells.
These wrinkles move through different groups of stars in the Milky Way and affect how the stars move through space. By measuring the positions and velocities of these stars with great precision, Gaia can detect the wrinkles, the imprint of the last major merger.
“Our wrinkles diminish as we age, but our work reveals that the opposite is true for the Milky Way. It’s a sort of cosmic Benjamin Button that shrinks over time,” lead author Donlon said 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 that it happened billions of years later than we thought.”
The quest to understand the last major merger of the Milky Way (MW) involves a variety of lines of evidence. One piece of evidence, along with the wrinkles, is the Fe/H-rich region where the stars follow a highly eccentric path.
A star’s Fe/H ratio is a chemical fingerprint, and when astronomers find a group of stars with the same fingerprint and the same orbits, it’s evidence of a common origin. This group of stars is sometimes called a ‘splash’. Stars in the Splash may originate from an Fe/H-rich progenitor. They have peculiar orbits that stand out from their surroundings. Astronomers think they were heated and their orbits changed as a byproduct of the merger.
There are two competing explanations for all the evidence of association.
One says that a progenitor dwarf galaxy called Gaia Sausage/Enceladus (GSE) collided with the MW protodisk 8 to 11 billion years ago. Another explanation is that an event called the Virgo Radial Merger (VRM) is responsible for the stars in the inner halo. The collision occurred much more recently, less than 3 billion years ago.
“These two scenarios make different predictions about the observable structure in the local phase space because the morphology of the debris depends on how long the phases had to mix,” the authors explain in their paper.
Wrinkles in the MW were first identified in Gaia data in 2018 and presented in this paper.
“We observed shapes with different morphologies, such as a spiral similar to a snail’s shell. The existence of these substructures was observed for the first time thanks to the unprecedented precision of the data brought by the Gaia satellite from the European Space Agency (ESA),” said Teresa Antoja, first author of the study, in 2018.
However, Gaia has released more data since 2018 and supports a newer merger scenario, the Virgo Radial Merger. These data show that wrinkles are much more common than earlier data and studies based on them.
“For the stars’ wrinkles to be as bright as they appear in the Gaia data, they must have joined us less than 3 billion years ago—at least 5 billion years later than previously thought,” said co-author Heidi Jo Newberg of Rensselaer Polytechnic Institute.
If the wrinkles were much older and consistent with the GSE merger scenario, they would be more difficult to discern.
“New star wrinkles form every time the stars swing back and forth through the center of the Milky Way. If they joined us 8 billion years ago, there would be so many wrinkles right next to each other that we wouldn’t be able to see.” as separate elements,” Newberg said.
This is not to say that there is no evidence of ancient GSE fusion. Some of the stars that indicate an ancient merger may be from the more recent VRM merger, and some may still be associated with the GSE merger.
It is difficult to figure out and simulations play a big role. The researchers in the previous work and in this work ran several simulations to see how they matched the evidence.
“Our goal is to determine the time that has passed since the progenitor of the local phase-space folds collided with the MW disk,” the authors write in their paper.
“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 the actual Gaia data of the Milky Way—the method we used in this one.” ” also a new study,” said Thomas.
“Thus, we found that the wrinkles were probably caused by the collision of a dwarf galaxy with the Milky Way about 2.7 billion years ago. We called this event the Virgo Radial Merger.” These results and title are from a previous study from 2019.
As Gaia brings in more data with each release, astronomers are getting a better look at evidence of mergers. It is becoming clear that MW has a complicated history.
A VRM probably involved more than one entity. It could have brought an entire group of dwarf galaxies and star clusters into the MW at about the same time. As astronomers study the MW merger history in more detail, they hope to determine which of these objects come from the more recent VRM and which from the ancient GSE.
“The history of the Milky Way is constantly being rewritten at the moment, thanks in no small part to new data from Gaia,” adds Thomas. “Our picture of the Milky Way’s past has changed dramatically since a decade ago, and I think our understanding of these mergers will continue to change rapidly.”
“This finding improves what we know about the many complicated events that shaped the Milky Way and helps us better understand how galaxies form and are shaped – especially our home galaxy,” said Timo Prusti, project scientist for Gaia at ESA.
This article was originally published by Universe Today. Read the original article.