The Milky Way is only as massive as a result of collisions and mergers with other galaxies. This is a complicated process, and we see the same thing happening with other galaxies across the Universe.
Currently, we see the Milky Way nibbling away at its two satellite galaxies, the Large and Small Magellanic Clouds. Their fate is likely sealed and they will be absorbed into our galaxy.
Researchers believed the last major merger occurred in the Milky Way’s distant past, between 8 and 11 billion years ago. But new research amplifies the idea that it was much younger: less than 3 billion years ago.
This new insight into our galactic history comes from ESA’s Gaia mission. Launched in 2013, Gaia currently maps 1 billion astronomical objects, mainly stars. He measures them repeatedly, establishing precise measurements of their positions and movements.
A new article published in the Monthly Notices of the Royal Astronomical Society presents the results. It’s called “The debris of the ‘last major merger’ is dynamically young.” » The lead author is Thomas Donlon, a postdoctoral researcher in physics and astronomy at the University of Alabama in Huntsville. Donlon has studied mergers in the Milky Way for several years and has published other work on the subject.
Every time another galaxy collides and merges with the Milky Way, it leaves ripples. “Wrinkles” is obviously not a scientific term. It is a generic term for several types of morphologies, including phase space folds, caustics, chevrons, and shells.
These ripples move through different groups of stars within the Milky Way, affecting the way stars move through space. By measuring the positions and speeds of these stars with great precision, Gaia can detect the wrinkles, the mark of the last major merger.
“We become more wrinkled with age, but our work reveals that the opposite is true for the Milky Way. It’s a kind of cosmic Benjamin Button, becoming less wrinkled with time,” said lead author Donlon in a press release.
“By observing how these wrinkles dissipate over time, we can trace the moment when the Milky Way experienced its last big crash – and it turns out that it happened billions of years later than expected. “
The effort to understand the last major merger of the Milky Way (MW) involves different lines of evidence. One piece of evidence, besides the wrinkles, is an Fe/H-rich region where the stars follow a very eccentric orbit.
A star’s Fe/H ratio is a chemical fingerprint, and when astronomers find a group of stars with the same fingerprint and orbits, it’s evidence of a common origin. This group of stars is sometimes called “the Splash.” The Splash stars could come from an Fe/H-rich ancestor. They have strange orbits that stand out from their surroundings. Astronomers believe they were heated and their orbits changed as a result of the merger.
There are two competing explanations for the body of evidence for fusion.
One says that a progenitor dwarf galaxy named Gaia Sausage/Enceladus (GSE) collided with the MW proto-disk between 8 and 11 billion years ago. The other explanation is that an event called Virgo Radial Merger (VRM) is responsible for the stars in the inner halo. This collision occurred much more recently, less than 3 billion years ago.
“These two scenarios make different predictions about the structure observable in local phase space, because the morphology of the debris depends on the length of time it mixes,” the authors explain in their paper.
Wrinkles in the MW were first identified in Gaia data in 2018 and presented in this article.
“We observed shapes with different morphologies, such as a spiral similar to a snail shell. The existence of these substructures was observed for the first time thanks to the unprecedented precision of the data provided by the Gaia satellite, of the European Space Agency (ESA),” Teresa Antoja, the study’s first author, said in 2018.
But Gaia has released more data since 2018 and supports the most recent merger scenario, the Virgo Radial Merger. This data shows that wrinkles are much more prevalent than previous data and studies based on them suggest.
“For star wrinkles to be as clear as they appear in Gaia’s data, they must have reached us less than 3 billion years ago, or at least 5 billion years later than we thought. thought so before,” 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 harder to discern.
“New star ripples form every time the stars rock back and forth in the center of the Milky Way. If they had joined us 8 billion years ago, there would be so many ripples there next to each other that we would no longer see them as separate features,” Newberg said.
This is not to say that there is no evidence of the older GSE merger. Some of the stars that hint at the older merger could come from the more recent VRM merger, and some could still be associated with the GSE merger.
It’s difficult to understand and simulations play an important role. Researchers in the previous work and this work ran several simulations to see how they matched the evidence.
“Our goal is to determine how much time has passed since the ancestor of local phase space folds collided with the MW disk,” the authors write in their paper.
“We can see how the shapes and number of wrinkles change over time with these simulated mergers. This allows us to determine the exact moment when the simulation best matches what we see today in the real data of Milky Way Gaia – a method we used in this new study as well,” Thomas said.
“By doing this, we discovered that the ripples were likely caused by the collision of a dwarf galaxy with the Milky Way about 2.7 billion years ago. We named this event the Virgo radial merger. ” These results and the name come from a previous 2019 study.
As Gaia provides more data with each release, astronomers get a better look at the evidence for mergers. It becomes clear that the MW has a complex history.
The VRM likely involved more than one entity. This could have brought a whole group of dwarf galaxies and star clusters into the MW at around the same time. As astronomers study the MW’s merger history in more detail, they hope to determine which of these objects came from the newer VRM and which came from the older GSE.
“The history of the Milky Way is currently being constantly rewritten, largely thanks to new data from Gaia,” adds Thomas. “Our picture of the Milky Way’s past has changed dramatically over the past decade, and I think our understanding of these mergers will continue to evolve rapidly.”
“This discovery improves what we know about the many complex events that shaped the Milky Way, helping us better understand how galaxies form and shape – our home galaxy in particular,” said project scientist Timo Prusti. Gaia at ESA.
This article was originally published by Universe Today. Read the original article.