The Milky Way as we know it today was formed by a collision with a dwarf galaxy about 10 billion years ago. But most of the modern galaxy was already in place even at that early date, according to new research.
The ages of the stars left by the galactic interloper are a little younger or on par with the stars on the main disk of the Milky Way, researchers report on May 17 in Nature Astronomy. And that could mean the Milky Way grew faster than astronomers expected, says study author Ted Mackereth, an astrophysicist at the University of Toronto.
“The Milky Way had already accumulated long before this great merger took place,” he says.
The history of our galaxy is one of violent conquest. Like other giant spiral galaxies in the universe, the Milky Way probably accumulated its largest volume by colliding and merging with smaller galaxies over time. The stars of the unfortunate devoured galaxies mingled in the Milky Way like coffee cream, making it difficult to understand what the galaxies were like before they merged.
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In 2018, astronomers realized they could identify stars from the latest major fusion using detailed maps of several million stars from the European Space Agency’s Gaia spacecraft (SN: 5/9/18). Star currents orbit around the galactic center at an angle to the main disk of the stars. The movements and chemistry of these stars suggest that they belonged to a separate galaxy that sank in the Milky Way about 10 billion years ago (SN: 01/11/2018).
“Those stars stay there like fossil remains of the galaxy,” Mackereth says.
Two groups discovered evidence of the ancient galaxy at the same time. One was called the Gaia-Enceladus galaxy; the other group called it Sausage. The remaining name was Gaia-Enceladus / Sausage.
Mackereth and his colleagues wondered if they could find out how well-developed the Milky Way was when Gaia-Enceladus / Sausage collided. If the oldest stars on the Milky Way disk formed after this fusion, they would probably have formed as a result of this fusion. collision, suggesting that Gaia-Enceladus / Sausage found a proto-Milky Way that still had much to do. On the other hand, if the oldest stars are about the same age or older than the stars of the galactic interloper, then our galaxy was probably quite well developed at the time of the run over.
Previous researchers had made estimates. But Mackereth and colleagues used an accurate tool called asteroseismology to discover the ages of individual stars in both the Milky Way and Gaia-Enceladus / Sausage (SN: 8/2/19). Just as Earth’s seismologists use earthquakes to probe the interior of our planet, asteroseismologists use variations in brightness caused by earthquakes and other oscillations to probe the bowels of stars.
"Asteroseismology is the only way to access the inner part of stars," says study physicist and co-author Josefina Montalbán of the University of Birmingham in England. From information about the inner structures of the star, researchers can deduce the age of the stars.
The team selected about 95 stars that had been observed by the Kepler space telescope from NASA's exoplanet hunt, which completed its mission in 2018 (SN: 30/10/18). Six of those stars were from Gaia-Enceladus / Sausage, and the rest were from the thick disk of the Milky Way. Measuring how the brightness of those stars fluttered over time, Mackereth and his colleagues deduced ages with an accuracy of about 11 percent.
The Gaia-Enceladus / Sausage stars are slightly younger than the Milky Way roads, but they were all fairly close to 10 billion years old, the team found. This suggests that a large chunk of the Milky Way disk was already in place when Gaia-Enceladus / Sausage collided. It is still possible that the incoming galaxy caused the formation of some new stars, though, Mackereth says. To say how much, they will have to get ages of many more stars.
Measuring ages for individual stars is a step forward for galactic astronomy, says astrophysicist Tomás Ruiz-Lara of the University of Groningen, the Netherlands, who studies galactic evolution but did not participate in the new work.
“If you don’t know how to differentiate between a child, a teenager, and an adult, then we can’t say anything” about a population of people, Ruiz-Lara says. "But if I can distinguish someone between 40 or 50, you have a better graph of society. With stars, it's the same. If we're able to distinguish age correctly, then we can distinguish individual events in the history of the galaxy. In the end, that is the goal ".