New research provides the best evidence to date into the timing of how our
early Milky Way came together, including the merger with a key 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 galaxy.
With this and other data, the researchers were able to show what was
happening when the Milky Way merged with an orbiting satellite galaxy, known
as Gaia-Enceladus, about 10 billion years ago.
Their results were published today (May 17, 2021) in the journal Nature
Astronomy.
“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.
Many of those “homemade” stars ended up in the thick disc in the middle of
the galaxy, while most that were captured from Gaia-Enceladus are in the
outer halo of the galaxy.
“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,” said
Josefina Montalban, with the School of Physics and Astronomy at the
University of Birmingham in the U.K., 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 have similar
or slightly younger ages compared to the majority of stars that were born
inside the Milky Way.
A violent merger between two galaxies can’t help but shake things up,
Vincenzo said. Results showed that the merger changed the orbits of the
stars already in the galaxy, making them more eccentric.
Vincenzo compared the stars’ movements to a dance, where the stars from the
former Gaia-Enceladus move differently than those born within the Milky Way.
The stars even “dress” differently, Vincenzo said, with stars from outside
showing different chemical compositions from those born inside the Milky
Way.
The researchers used several different approaches and data sources to
conduct their study.
One way the researchers were able to get such precise ages of the stars was
through the use of asteroseismology, a relatively new field that probes 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 happened in the early Milky
Way,” Vrard said.
The study also used a spectroscopic survey, called APOGEE, which provides
the chemical composition of stars – another aid in determining their ages.
“We have shown the great potential of asteroseismology, in combination with
spectroscopy, to age-date individual stars,” Montalban said.
This study is just the first step, according to the researchers.
“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.”
Reference:
Josefina Montalbán, J. Ted Mackereth, Andrea Miglio, Fiorenzo Vincenzo,
Cristina Chiappini, Gael Buldgen, Benoît Mosser, Arlette Noels, Richard
Scuflaire, Mathieu Vrard, Emma Willett, Guy R. Davies, Oliver J. Hall,
Martin Bo Nielsen, Saniya Khan, Ben M. Rendle, Walter E. van Rossem, Jason
W. Ferguson, William J. Chaplin. Chronologically dating the early assembly
of the Milky Way. Nature Astronomy, 2021; DOI:
10.1038/s41550-021-01347-7
Tags:
Space & Astrophysics