Recent research by OSU astronomers reveals new estimates of when the Milky Way formed. The findings also provide a cosmological timescale of when our galaxy merged with other, smaller galaxies. Using new astronomy techniques, the researchers identified the most accurate ages of nearly 100 red giant stars in the Milky Way.
This, along with other data, revealed evidence of what happened when the Milky Way merged with an orbiting satellite galaxy. The galaxy, known as Gaia-Enceladus, went through a violent merger with our galaxy about 10 billion years ago. The journal Nature Astronomy published the findings on May 17, 2021.
“Our evidence suggests that when the merger occurred, the Milky Way had already formed a large population of its own stars,” said Fiorenzo Vincenzo, co-author of the study and a fellow in The Ohio State University’s Center for Cosmology and Astroparticle Physics.
Most of the stars in the Milky Way appeared in the thick disk in the middle of the galaxy. On the other hand, most stars formed in the outer halo of the Gaia-Enceladus galaxy.
Josefina Montalban, with the School of Physics and Astronomy at the University of Birmingham in the U.K. and the lead author of the study, says this:
“The merging event with Gaia-Enceladus is thought to be one of the most important in the Milky Way’s history, shaping how we observe it today.”
Calculating the age of the stars allowed the researchers to identify new information about both galaxies for the first time. They discovered that the stars captured from Gaia-Enceladus are very close in age to most stars born in the Milky Way.
Vincenzo says that anytime a violent merger occurs between two galaxies, significant changes happen. Their data showed that when the two galaxies collided, the orbits of the already-existing stars changed, making them more chaotic.
Methods used to find out how the Milky Way formed.
Vincenzo compared the stars’ erratic movements to a dance. He said the stars from the Gaia-Enceladus galaxy move differently than those within the Milky Way. Vincenzo described other differences between the stars in each galaxy. The stars from the former galaxy had different chemical compositions than those born within the Milky Way.
For the study, the team utilized various methods and data sources within astronomy. To identify the most precise ages of the stars, they used asteroseismology – the study of stellar oscillations. According to Mathieu Vrard, a postdoctoral research associate in Ohio State’s Department of Astronomy, these are sound waves that ripple through the internal structure of stars.
“That allows us to get very precise ages for the stars, which are important in determining the chronology of when events happened in the early Milky Way,” Vrard said.
The study also utilized data from a spectroscopic survey called APOGEE. This reveals the chemical composition of stars, which also aids in dating the stars.
“We have shown the great potential of asteroseismology, in combination with spectroscopy, to age-date individual stars,” Montalban said.
The research team says these findings mark the first step in uncovering more information about galaxies in the future.
“We now intend to apply this approach to larger samples of stars and to include even more subtle features of the frequency spectra,” Vincenzo said.
“This will eventually lead to a much sharper view of the Milky Way’s assembly history and evolution, creating a timeline of how our galaxy developed.”