New research from astronomers shows that the outer edge of the region of the Milky Way that is responsible for the birth of new stars may be closer to the center of the Milky Way than previous models predicted. By accurately determining the ages of more than 100,000 giant stars, an international research team has clearly defined the "boundary" of the Milky Way's star-forming disk for the first time, finding that the region where recent star formation activity occurred does not extend as far as people expected.
Existing galaxy evolution models generally believe that new stars will be born in a "relay" from the inside to the outside of the galaxy disk, so as the distance from the galactic center increases, the average age of stars should gradually become smaller. However, the team saw two completely different age trends in the observational data: In the inner disk region of the Milky Way, stars get younger as they move outward; but when they are about 40,000 light-years away from the center of the Milky Way, this trend suddenly reverses, and the outer stars become older. The result is a "U"-shaped age curve, with the youngest stars concentrated around a certain radius. This structure is seen as a distinct marker for the outer edge of the Milky Way's star-forming disk.
Karl Fiteni, the first author of the paper and an astrophysicist at the University of Insubria, said that how far the Milky Way's star-forming disk extends has always been an open question in "galactic archeology". Now, by mapping the fine distribution of star age as a function of radius, researchers have finally given a quantitative and clear answer. The study used two major stellar survey data: LAMOST-DR3 and APOGEE-DR17, combined with AstroNN neural network distance estimation and Gaia high-precision astrometric data. The sample selection was mainly limited to stars close to the midplane of the galactic disk and with highly circular orbits to highlight the intrinsic properties of the disk itself as much as possible.
The researchers combined the ages of giant stars with numerical simulation results to draw a "fingerprint" of the age of stars in the Milky Way as a function of radius, which clearly shows that there is a significant structural boundary at about 35,000 to 40,000 light-years. This feature is very stable in different survey data and has nothing to do with the data set used. The corresponding radius is also highly consistent with the so-called "breaking radius" where the star density profile in the galaxy disk is obviously "broken", which is regarded as the physical edge of the star forming disk.
Co-author Joseph Caruana, an astrophysicist at the University of Malta, pointed out that the high-precision stellar age data available today is becoming a powerful tool for interpreting the history of the Milky Way, pushing us into a "new era" of using stellar ages to reconstruct the evolution history of our own galaxy. Beyond this disk edge, star formation activity has significantly attenuated, and the disk mass density continues to decrease, but observations still reveal the presence of a large number of stars, which raises a key question: If new stars are almost no longer formed in the outer disk, how do these stars appear there?
The answer given by research is "radial migration." Stars can slowly "drift" outward in the galactic disk. This process is vividly likened to "surfing" on the spiral wave of the galactic disk: stars are like surfers using the waves to reach the shore, grabbing the spiral arms that pass through the galaxy, guiding themselves to gradually leave their birthplace and move further outward. Since this migration is slow and random, the farther away it is, the longer it takes for stars to complete their migration. Therefore, stars with the highest average age are gathered on the outermost side of the area away from the age "trough".
Observations and simulations show that this "breaking radius" is not caused by statistical biases such as assumed differences in the sun's position or insufficient sample sizes in other surveys, but is the real physical boundary of the Milky Way's disk structure. This result supports the view that the Milky Way is a typical type II (downward curve) disk galaxy, that is, outside the break radius, the number of stars is more abundant than in the simple exponential disk model. This structure is believed to originate from the competition between star formation truncation and radial migration, and leaves a "U" shaped evolutionary fossil record in the star age distribution.
Relevant research not only further refines our understanding of the formation and evolution of the Milky Way, but also provides an important reference rule for understanding other disk galaxies. The relatively "quiet" outer disk of the Milky Way in the traditional view has now been re-depicted as a dynamic region that evolves under the joint action of radial migration, orbital resonance and gradually decaying star formation. Its complex gravitational interactions continue to reshape this galaxy space that was once considered "edge and bland".