Disk galaxies like the Milky Way form stars “inside-out” — starting from the center and working outwards through the disk. So, as a general rule, the farther out astronomers look, the younger the stars are.
Now, a team led by Karl Fiteni (then at University of Malta), carried out under the supervision of Joseph Caruana and Victor Debattista, has analyzed more than 100,000 giant stars. By coupling observations with advanced computer simulations, the astronomers show that this inside-out pattern reverses at between 35,000 and 40,000 light-years from the Milky Way’s center. Beyond this distance, the stars are older again.
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The analysis involves data from the LAMOST and APOGEE spectroscopic surveys, as well as measurements from the European Space Agency’s Gaia satellite. Fiteni’s team focused on red giant branch stars, whose ages can be estimated with relatively high precision. The results are published in Astronomy & Astrophysics.
The stars beyond this boundary probably weren’t formed in situ. But they didn’t come from infalling satellite galaxies either. Instead, they likely migrated outward over time. “A key point about the stars in the outer disk is that they are on close-to-circular orbits, meaning that they had to have formed in the disk,” says team member Victor Debattista (University of Lancashire, UK).
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Similar U-shaped age profiles have been seen in simulated disk galaxies and inferred in observations of other galaxies beyond our own. So the Milky Way is not unusual but merely following a common pattern of disk evolution, with the newly identified boundary marking a transition that may be a generic feature of spiral galaxies.
It is currently unclear what stymies star formation beyond this boundary. It is possible that the gravity of the Milky Way’s central bar corrals gas at preferred radii. Or it could be due to the bend of the galaxy, which warps towards the edge, disrupting star formation in the outer reaches.
New and future instruments could help paint a clearer picture. They include the 4MOST spectroscopic instrument at the European Southern Observatory in Chile, which saw first light last October, and the WEAVE spectrograph, attached to the William Herschel Telescope at La Palma in the Canary Islands.
From the article:
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