Astronomers have pinpointed the edge of the Milky Way's star-forming disk for the first time. The latest research shows that the star formation activity of the Milky Way is mainly concentrated within about 40,000 light-years from the galactic center. Although stars still exist outside this range, most of them are not newly born locally, but are more likely to have gradually migrated from the interior of the Milky Way.
According to the general evolution rules of disk galaxies, star formation usually exhibits the characteristics of "inside-out" advancement: the galaxy first forms stars in the central region, and then the star formation area gradually expands outward. Therefore, generally the farther away from the center, the younger the star. The study, led by Carl Fitney and completed under the guidance of Joseph Caruana and Victor DeBatista, analyzed more than 100,000 giant stars and combined with advanced computer simulations found that this pattern reverses in a region about 35,000 to 40,000 light-years from the galactic center: beyond this distance, stars age again.
The research team pointed out that this change formed a typical "U-shaped age distribution." The area corresponding to the lowest point of the curve represents a sharp decline in star formation activity, and is therefore considered the boundary of the Milky Way's star-forming disk. Researchers said that how far the Milky Way's star-forming disk extends has long been an important unsolved question in "galactic archeology". By mapping the distribution of stellar age changes along the galactic disk, this question has finally obtained a clearer and quantifiable answer.
In terms of data sources, this study comprehensively used two spectral survey data, LAMOST and APOGEE, 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 such stars can be estimated with high accuracy. Relevant results have been published in "Astronomy & Astrophysics".
As for the stars outside the boundary, researchers believe that they are most likely not produced in situ, but they are not merging inputs from alien dwarf galaxies or satellite galaxies. A more reasonable explanation is that these stars originally formed in the inner disk of the Milky Way and then gradually migrated outward over a long period of time. Victor DeBattista, a member of the research team, pointed out that most of the outer disk stars move in nearly circular orbits, which means that they must have formed in the galactic disk itself, rather than from an external system.
Researchers describe this process as being like a surfer being pushed to the shore by a wave: the density waves stirred up by the spiral arms of the Milky Way will continue to push stars outward, and eventually "transport" them to further outer regions. Since it takes longer to migrate to farther locations, the outermost stars tend to be the oldest. It is this outward migration mechanism that causes the age of stars to rise again beyond the edge of the star forming disk.
In fact, similar U-shaped age distributions have previously appeared in simulations of disk galaxies, and have also been indirectly inferred from observational studies of other galaxies. This means that the Milky Way is not a special case, but follows a more general evolution pattern of disk galaxies; the boundary identified this time is likely to correspond to a universal turning structure in the evolution of spiral galaxies.
However, the mechanism that hinders star formation beyond this boundary is still unclear. The researchers proposed two possible explanations: first, the gravity of the central bar structure of the Milky Way may constrain the gas within a specific radius; second, there is an obvious warp in the edge region of the Milky Way, and this curved structure may disrupt the star formation process in the outer regions.
In the future, a new generation of observation equipment is expected to help astronomers further clarify this issue. These include the European Southern Observatory's 4MOST spectrograph - which saw its "first light" last October - and the WEAVE spectrograph installed on the William Herschel Telescope on La Palma, Canary Islands. As the study of galactic archeology continues to advance, scientists hope to not only gain a deeper understanding of the past and future of the Milky Way, but also to explain the evolution of more similar galaxies.