A new way to determine the ages of stars in the Milky Way narrows down the possibilities for a galactic collision.
University of Birmingham astronomers analyzed 95 red giant stars closely to determine that the Milky Way punched on with a mysterious galaxy called Gaia-Enceladus, or the Gaia Sausage, around 10 billion years ago.
The researchers also found that many of the stars in the Milky Way had already formed before this merger. Now, these original stars are mostly in the thick disk of the galaxy, the larger of the Milky Way's two disk structures.
"The chemical composition, location and motion of the stars we can observe today in the Milky Way contain precious information about their origin," explained astronomer Josefina Montalbán of the University of Birmingham in the UK.
"As we increase our knowledge of how and when these stars were formed, we can start to better understand how the merger of Gaia-Enceladus with the Milky Way affected the evolution of our galaxy."
Illustration of the young Milky Way merging with Gaia-Enceladus around 10 billion years ago (left) and today's Milky Way (right).
Over the last 13.6 billion years, the Milky Way has been through a lot. It's eaten smaller galaxies, stealing their stars.
These collisions were revealed by the ongoing Gaia survey, which maps the Milky Way in 3D, so that, as far as we can tell, the Gaia Sausage was the biggest of the galaxies slurped up by the Milky Way.
Those stars can tell you a lot, but you need more than just their motions to get the full picture. Knowing the ages and chemical compositions can also help determine where the stars came from.
You can tell the age of a star by its chemical composition - younger stars have more heavy elements, or higher metallicity, since metals didn't exist before stars created them via nuclear fusion and supernovae. Spectroscopy shows what's in the composition by looking for changes in wavelengths.
A spectroscope is a pretty good tool, but for even higher precision, the research team turned to astero-seismology.
Astero-seismology studies the oscillation frequencies of stars, or pulsations in their light intensity caused by internal acoustic waves. These oscillations are closely connected to the properties of the star's interior, such as its density and acoustic profile, which are linked to its mass and age.
Kepler, now retired, was optimized to look for star brightness changes, one way to find exoplanets orbiting other stars. Researchers collected astero-seismological data on hundreds of stars; Montalbán and her team chose a few red giant stars with low metallicity since these are long-lived and intrinsically bright, making them perfect for mapping star ages.
As a result, they examined 95 of these, focusing on the properties of each star instead of averaging them out.
A vaguely accurate depiction of the Gaia sausage (banana) inserting itself into the Milky Way. (No bananas were harmed during the filming of this gif.)
Aspectroscopy revealed that some of the stars originated in the Gaia sausage, and asteroseismology revealed they were all about the same age, about 10 billion years old, or a bit younger, than the stars that formed in our Milky Way galaxy.
The Milky Way gave birth to quite a few stars before it merged with the Gaia Sausage, and it was doing so efficiently. Scientists said this raises the possibility that Milky Way-Gaia Sausage infall occurred around 10 billion years ago.
"We have shown the huge potential of asteroseismology in combination with spectroscopy to deliver precise, accurate relative ages for individual, very old, stars," said astronomer Andrea Miglio, of the University of Bologna in Italy.
"Taken together, these measurements contribute to sharpen our view on the early years of our Galaxy and promise a bright future for Galactic archeoastronomy."
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