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About 10 billion years ago, the Milky Way – then small galaxy that did not contain the current spiral structure or the diffuse halo of surrounding stars – suffered a massive frontal collision that l? shook up to its core.
It was at this point that the gravity of our galaxy dragged a small companion, about a quarter of its mass, into a dangerous dance: the one in which the dwarf galaxy plunged into and out of the Milky Way disk, oscillating until it is finally swallowed whole. Although our galaxy has survived, it has never been the same. The collision blurred the stars' orbits into his disk, making him much more puffy, and sent alien stars flying all around the Milky Way, building up much of his halo. The smash up also channeled new gases to the galactic center, adding fuel mixed with the existing tanks of the Milky Way to form new generations of stars.
Over time, the dwarf galaxy has faded, but the scars of its collision have never really disappeared – though they were easy to find. Astronomers, who have long believed that the Milky Way probably originated from a large number of dwarf galaxies that merge, have struggled to discover the signs of the largest mergers – until today. . Now, a new article published in Nature provides proof or something close. "It's like discovering a fossil or archaeological element that proves the beginning of the galaxy," says James Bullock, an astronomer at the University of California, Irvine, who is not affiliated with the new research.
Co-written by Amina Helmi, astronomer at the Kapteyn Institute of Astronomy in the Netherlands, this article is part of a torrent that follows the long-awaited second publication of Gaia, a probe launched in 2013 by the European Space Agency. to map the heavens with unprecedented details. During his mission, Gaia pinpointed with such precision the positions, movements, luminosities and colors of the 1.3 billion stars of the Milky Way that astronomers used his data to create new chapters of an exceptional richness in the Milky Way's biography.
Indeed, after the release of the second dataset in April, many teams glimpsed allusions to our violent story – recorded in stars that seemed out of place. Vasily Belokurov, astronomer at the University of Cambridge, led a team that discovered a large number of stars out of sync with the rotation of the galaxy, which were heading towards the galactic center rather than against all expectations. Another Kapteyn astronomer, Helmer Koppelman, led a study that spotted a "cloud" of stars in orbit in the opposite direction to most stars in the Milky Way halo. And Misha Haywood, astronomer at the Paris Observatory, found fast stars in the halo with an atypical chemical abundance – a sign that they could have formed outside the Milky Way. "It was really the tip of the iceberg," says Helmi. Although all of these studies have attributed such strangeness to a past collision, they disagreed when the collision occurred, on the mass of the satellite galaxy and on the fact that the event involved only one large dwarf galaxy or several smaller ones. Helmi's study, on the other hand, brings together several data sources to paint the most convincing picture to date, says Kathryn Johnston, an astronomer at Columbia University, who did not participate in this work. And it's a strategy that relies on a unique but massive merger.
Helmi and his colleagues analyzed 50,000 Gaia stars in the Milky Way halo, and found that 33,000 of them shared similar amounts of kinetic momentum and gravitated in the opposite direction. "That's when it became obvious that it was weird," she says. "The disc is ordered; you have 100 million stars moving in order around the galactic center. And then you have those stars that have decided to move in the opposite direction. This suggests that these stars could not have been formed in the Milky Way. To further test this hypothesis, the team also analyzed the chemical compositions of 600 of these previously studied stars with the APOGEE ground star survey. In each galaxy, the abundance of elements heavier than hydrogen and helium increases gradually over time due to stellar death and rebirth cycles. But the exact pattern of this increase is specific to each galaxy, much like a fingerprint. Taken together with simulations modeling the potential fusion, the data of Gaia and APOGEE reinforce the notion that these stars are real intruders, suggesting that they come from one and the same big dwarf galaxy that stopped forming stars. 10 billion years ago – when it was cannibalized.
The team dubbed the dead galaxy "Gaia – Enceladus", in honor of the space observatory and Greek mythology Enceladus, a giant who would have been buried under the Etna volcano and responsible for the earthquakes In the region.
The study, if correct, first confirms what theorists have long thought: that galaxies such as the Milky Way reach enormous proportions by devouring many smaller ones. "We did not know if the Milky Way had been merged," says Helmi. "And you never know when you see a merger in another galaxy, if it's just anecdotal.So the fact that we now have the data, and that tells us that mergers have occurred and have had a significant impact on the history of our galaxy – I think this is a very important step to confirm this picture: we have galaxies build through mergers. "This confirmation alone has sent many astronomers in a state of euphoria.
In addition, the study also offers an unprecedented opportunity for new avenues of research. Although astronomers have seen galaxies melt into distant corners of the cosmos, a collision inside the Milky Way – even if it remains a remnant – provides a front-row seat that delivers well more answers. Even our closest neighbor, Andromeda, is too far away to conduct in-depth studies on mergers in this country. "Less than three million light years away, we never go to Andromeda to populate it or study it in detail," says Kim Venn, an astronomer at the University of Victoria in British Columbia, who did not participate in the study. "This is our only chance." And with this chance, astronomers could better analyze the physics involved. Let's take the example of the spiral disk of stars of the Milky Way. This disc is composed of two parts: a thinner and denser region surrounded by a thicker and more diffuse region – but no one knows how the thick disc appeared. Although Helmi notes that the collision in question heated and blew part of the thin disc into a thick disc, future simulations of the collision will help astronomers to better answer this question.
And the answer will only shed light on these galactic wrecks. Helmi wants to use the evolution of the Milky Way "as if it were a Rosetta stone" to better understand the evolution of the galaxy in the universe and its impact on the cosmos. The results could even help astronomers to imagine a future collision near their home, Venn said, when the Andromeda galaxy will hit the Milky Way directly in about four billion years ago.
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