Galaxy's "Boundary" Located for the First Time: Star Formation Disk Shrinks Significantly About 40,000 Light-Years from the Center
Astronomers have, for the first time, relatively clearly located the edge of the Milky Way's star formation disk. The latest research shows that the galaxy's star formation activity is mainly concentrated within about 40,000 light-years of the galactic center. Beyond this range, while stars still exist, most were not recently born locally but likely migrated gradually from within the galaxy.
Following the general evolutionary patterns of disk galaxies, star formation typically exhibits a characteristic of advancing "from inside to outside": galaxies first form stars in the central region, and then the star formation region gradually expands outward. Therefore, the farther away from the center, the younger the stars generally are. This research, led by Carl Fittnes and supervised by Joseph Caruana and Viktor Debattista, analyzed over 100,000 giant stars and, combined with advanced computer simulations, discovered that this pattern reverses in the region about 35,000 to 40,000 light-years from the galactic center: beyond this distance, the age of stars actually becomes older again.
The research team pointed out that this change forms a typical "U-shaped age distribution." The region corresponding to the lowest point on the curve indicates a sharp decline in star formation activity, and is therefore considered the boundary of the Milky Way's star formation disk. Researchers say that how far the Milky Way's star formation disk extends has long been an important unsolved problem in "galactic archaeology," and by mapping the distribution of star ages along the galactic disk, this question has finally received a relatively clear and quantifiable answer.
In terms of data sources, this study comprehensively used data from the LAMOST and APOGEE spectroscopic surveys, as well as measurements from the European Space Agency's Gaia satellite. The research objects are mainly red giant branch stars, because the age of these stars can be estimated with relatively high precision. The relevant results have been published in *Astronomy & Astrophysics*.
Regarding the stars beyond the boundary, researchers believe they were likely not formed in place, but are not from the merger input of external dwarf galaxies or satellite galaxies. A more reasonable explanation is that these stars were initially formed within the Milky Way's disk and then gradually migrated outward over a long period of time. Viktor Debattista, a member of the research team, pointed out that most outer disk stars orbit in nearly circular orbits, which indicates that they must have formed within the disk itself, rather than from external systems.
Researchers describe this process as somewhat like surfers being pushed ashore by waves: density waves excited by the Milky Way's spiral arms continuously push stars outward and eventually "transport" them to more outer regions. Because migrating to farther positions takes longer, the outermost stars are therefore often the oldest. It is this outward migration mechanism that causes the age of stars to rise again beyond the edge of the star formation disk.
In fact, similar U-shaped age distributions have previously appeared in disk galaxy simulations and have also been indirectly inferred in observational studies of other galaxies. This suggests that the Milky Way is not a special case, but follows a more common evolutionary pattern of disk galaxies; the boundary identified this time is likely to correspond to a common turning structure in the evolution of spiral galaxies.
However, what mechanism hinders star formation beyond this boundary remains inconclusive. Researchers propose two possible explanations: first, the gravity of the Milky Way's central bar structure may constrain gas within a certain radius; second, there is a significant warp in the outer regions of the Milky Way, and this curved structure may disrupt star formation in the outer regions.
In the future, a new generation of observational equipment is expected to help astronomers further clarify this issue. These include the 4MOST spectrograph of the European Southern Observatory – which had its "first light" last October – and the WEAVE spectrograph installed on the William Herschel Telescope on La Palma Island in the Canary Islands. As galactic archaeology research continues to advance, scientists hope to not only gain a deeper understanding of the Milky Way's past and future, but also use it to explain the evolutionary history of more similar galaxies.