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Best evidence yet of how and when the Milky Way came together

Aging of individual stars helped date an early fusion.

New research provides the best evidence yet on the timing of how our early Milky Way came about, including its merger with a major satellite galaxy.

Using relatively new methods in astronomy, the researchers were able to identify the most precise ages currently possible for a sample of about a hundred red giant stars in the Milky Way.

With this and other data, the researchers were able to show what happened when the Milky Way merged with an orbiting satellite galaxy known as Gaia-Enceladus about 10 billion years ago.

Their results were published in the journal Nature Astronomy on May 17, 2021.

“Our evidence suggests that by the time the fusion took place, the Milky Way had already formed a large population of its own stars,” said Fiorenzo Vincenzo, co-author of the study and a fellow at The Ohio State University Center for Cosmology and Astroparticle Physics. 

Many of those “homemade” stars have ended up in the thick disk in the center of the Milky Way, while most of Gaia-Enceladus are in the outer halo of the Milky Way.

“The union with Gaia-Enceladus is considered one of the most important in the history of the Milky Way and has shaped how we perceive it today,” said Josefina Montalban of the School of Physics and Astronomy at the University of Birmingham in the UK. , who led the project.

By calculating the age of the stars, the researchers were able to determine for the first time that the stars captured from Gaia-Enceladus are of a similar or slightly younger age compared to the majority of stars born in the Milky Way.

A violent merger between two galaxies could shake things up, Vincenzo said. The results showed that the fusion changed the orbits of the stars already in the galaxy, making them more eccentric.

Vincenzo compared the movements of the stars to a dance, in which the stars of the former Gaia-Enceladus move differently from those born in the Milky Way. In fact, the stars ‘dress’ differently, Vincenzo said, with outside stars exhibiting different chemical compositions than those born in the Milky Way.

The researchers used a variety of approaches and data sources to conduct their research. One way the researchers were able to get such precise ages for the stars was through the use of asteroseismology, a relatively new field that examines the internal structure of stars.

Asteroseismologists study oscillations in stars, which are sound waves that ripple through their interiors, said Mathieu Vrard, a postdoctoral research associate in Ohio State’s Department of Astronomy.

“That allows us to get very precise ages for the stars, which are important in determining the chronology of when events took place in the early Milky Way,” Vrard said.

The study also used a spectroscopic survey called APOGEE that shows the chemical composition of stars – another aid in determining their age.

“We have demonstrated the great potential of asteroseismology, in combination with spectroscopy, to age individual stars,” said Montalban. According to the researchers, this research is only the first step.

“We now plan to apply this approach to larger samples of stars, including even more subtle features of the frequency spectra,” said Vincenzo.

“This will ultimately lead to a much sharper picture of the assembly history and evolution of the Milky Way, providing a timeline of how our galaxy has evolved.