The formation of our Milky Way Galaxy can be split up qualitatively into different assembly phases that resulted in its structurally different stellar populations: the halo and the disk components. Revealing a quantitative overall picture of Milky Way’s assembly requires a large sample of stars with very precise ages. In new research, astronomers at the Max-Planck Institute for Astronomy have analyzed such a sample using subgiant stars.
“To unravel the assembly history of the Milky Way Galaxy we need to learn how many stars were born when, from what material and on what orbits,” said Max-Planck Institute for Astronomy’s Dr. Maosheng Xiang and Dr. Hans-Walter Rix.
“This requires precise age determinations for a large sample of stars that extend to the oldest possible ages (around 14 billion years).”
“Subgiant stars, which are stars sustained by hydrogen shell fusion, can be unique tracers for such purposes, as they exist in the brief stellar evolutionary phase that permits the most precise and direct age determination, because their luminosity is a direct measure of their age.”
“Moreover, the chemical element compositions determined from the spectra of their photosphere surfaces accurately reflect their birth material composition billions of years ago.”
“This makes subgiants the best practical tracers of Galactic archaeology, even compared to main-sequence turn-off stars, whose surface abundances may be altered by atomic diffusion effects.”
“However, because of the short lifetime of their evolutionary phase, subgiant stars are relatively rare, and large surveys are essential to build a large sample of these objects with good spectra, which have not been available in the past.”
In their study, the astronomers used data from ESA’s Gaia space observatory and the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) in China to identify around 250,000 subgiant stars, covering a large spatial volume of the Milky Way.
They estimated the individual ages of these stars and found that they ranged from about 1.5 billion to 13.8 billion years old.
They then identified and characterized the origins of the different structural elements in the disk and halo stellar populations.
Their findings suggest that the formation of the thick disk had already begun around 13 billion years ago, only 800 million years after the Big Bang.
The authors calculate that the inner Galactic halo finished assembling — through the merger of the Gaia-Enceladus galaxy with the ancient Milky Way — two billion years later, thought to coincide with the time when most of the stars in the thick disk formed via a burst.
“The age, composition and motion of these stars highlight the dynamic processes involved in the formation of our Galaxy,” they said.
The team’s paper was publsihed in the journal Nature.
M. Xiang & H.W. Rix. 2022. A time-resolved picture of our Milky Way’s early formation history. Nature 603, 599-603; doi: 10.1038/s41586-022-04496-5
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