This galactic collision shaped the history of the Milky Way

As the Milky Way grew, took shape, and took care of its own business about 10 billion years ago, it suffered a massive frontal collision with another small galaxy. This cosmic cataclysm has changed the structure of the Milky Way forever, shaping the thick spirals that emerge from the supermassive black hole at the heart of the galaxy. Two new studies – one published in June, another still peer-reviewed – describe evidence of this unnoticed event

"It's a big step forward," said Elena D & # 39; Onghia. an astrophysicist at the University of Wisconsin who is not affiliated with the new research. "It's interesting because we can finally see what is the history of the Milky Way."

To uncover evidence of the collision so much later, astronomers have to work as galactic archaeologists, sifting through a myriad of information-a story consistent with the evidence available. Both research teams used data from the Gaia space telescope of the European Space Agency, which has spent years collecting exceptionally rich biographies of millions of stars, but also their brightness, their temperature , their age and composition. They essentially created high-resolution, multi-dimensional maps of the Milky Way and used these maps to find abnormal populations of ancient stars that seem to retain a memory of the long-standing collision. "Gaia's results really allow us to see things in the galaxy that we suspected to be there but that we had not seen," says Kathryn Johnston, an astrophysicist at Columbia University [19659008]. but the indications were inconclusive. A separate group of unique stars would have been a gift that they are intruders from elsewhere, but no evidence exists. The long-standing collision has so upset things that revealing stars have been scattered throughout the galaxy. "There is debris everywhere," said Vasily Belokurov, an astronomer from the University of Cambridge and one of the leaders of one of the two teams. "You are currently surrounded by these debris now."

He and his team found a large number of stars that do not move at the rate of rotation of the galaxy. Instead, they move in radial orbits, streaming to or out of the center of the galaxy. These stars are also rich in "metals" – the catch-all description that astronomers give to anything heavier than hydrogen, helium or lithium. The metal-rich stars probably descend from many previous generations of stars. They are the descendants of stars of an ancient galaxy that hit the Milky Way, their orbits always reflecting the strange trajectory of this cosmic agitator.

"If you throw a stone into a pond, these ripples will last for a while.A similar way, if you shake the disc of the Milky Way, even billions of years ago, it may take some time before this reaction settles down, "said Johnston

a quieter story without significant collisions – an impact of a small" dwarf "galaxy could indeed have Stars cloud like the ones we see today, they discovered.Their work was published online earlier this month in the Monthly Notices of the Royal Astronomical Society .

The other group, led by Amina Helmi, astronomer at the University of Groningen in the Netherlands, based her study on a more recent and larger dataset of Gaia and included a more detailed analysis of the chemical properties of the stars.The abundance of iron, produced by expl Supernova osions, compared to elements like magnesium, generated by massive but short-lived stars, give clues about the galaxy's history until nowadays. Helmi and his team used this data to conclude that the internal region of the Milky Way contains traces of debris from an ancient galactic impact. They called this ancient galaxy Gaia-Encelade

The collision could help solve a long-standing question about the structure of the Milky Way. The spiral star disk of the galaxy is composed of two parts: a thinner and denser region surrounded by a thicker and more diffuse region. Astronomers are not sure of the origin of this thick disc. Perhaps these stars have come from another galaxy, or are they thin disk stars that have interacted with each other and migrated outward over time. The work of Helmi and Belokurov suggest that instead, the Gaia-Encelade collision ejected thin-disc stars into the thick disc. "If this collision happened to the young Milky Way, it would damage the star disk, destroy it and send stars to high galactic heights," said Belokurov

. The two groups are unsure of the probable size of Gaia-Enceladus and the exact moment she fell into the Milky Way. And no one can say for sure how the disc of our galaxy is heated and inflated into a thicker disc. "We do not understand how much the impact is alone, but now we have a culprit" who could have created the thick disc, Johnston said. "What would be really exciting would be to look carefully into the record and trace this event and see if we are able to find a more direct effect that continues, an echo that remains."

Original story reprinted with permission of Quanta Magazine, an independent publication of the Simons Foundation whose mission is to improve public understanding of science by covering research developments and trends in mathematics and in physical and life sciences.