A new study provides the best evidence yet of how our early Milky Way galaxy formed, including a merger with a key satellite galaxy. Using a relatively new method in astronomy, the researchers were able to determine the most accurate ages currently possible for a sample of about a hundred red giants in the Milky Way.
Using these and other data, the researchers were able to show what happened when the Milky Way merged with an orbiting satellite galaxy called Gaia-Enceladus about 10 billion years ago. Their results were published in Nature Astronomy on May 17, 2021.
"Our evidence suggests that when the merger occurred, the Milky Way was already forming a large number of its own stars," said study co-author Fiorenzo Vincenzo, a research fellow at Ohio State University's Center for Cosmology and Astroparticle Physics.
Many of these "home-made" stars end up in the thick disk in the middle of the Milky Way, while most of the stars captured from Gaia-Enceladus are in the Milky Way's outer halo.
Josefina Montalban, from the School of Physics and Astronomy at the University of Birmingham, UK, who led the project, said: "The merger event with Gaia-Enceladus is considered to be one of the most important events in the history of the galaxy, shaping the way we see it today."
By calculating the ages of the stars, the researchers were able for the first time to determine that the stars captured from Gaa-Enceladus had similar or slightly younger ages than most stars born in the Milky Way.
Vincenzo said a violent merger between the two galaxies could not help but throw things into turmoil. The results showed that the merger altered the orbits of stars already in the Milky Way, making them more eccentric. Vincenzo likens the movement of stars to a dance, with stars from the pre-Gaia-Enceladus moving differently from those born within the Milky Way. These stars even "dress" differently, Vincenzo said, with stars from the outside showing a different chemical composition than stars born in the Milky Way.
The researchers used several different methods and data sources to conduct their study. One of the ways researchers were able to get such an accurate estimate of a star's age is by using asteroseismology, a relatively new field that probes the inner structure of stars.
Mathieu Vrard, a postdoctoral research associate in the Department of Astronomy at Ohio State University, says astroseismologists study oscillations in stars, which are sound waves ripping through their interior. "This allows us to get very precise star ages, which are important for determining the chronological sequence of events in the early galaxy," Vrard said.
The study also used a spectral survey called Apogee, which provides the star's chemical composition -- another aid in determining its age. "We've shown the great potential of asteroidology, combining it with spectroscopy to date individual stars," Montalban said.
The researchers say the study is just a first step. "We now plan to apply this method to a much larger sample of stars and include more subtle features of the frequency spectrum," Vincenzo said.
"This will ultimately lead to a clearer view of the history and evolution of the Milky Way, creating a timeline for how our galaxy developed."