The edge of the Milky Way’s star-forming disc: Evidence from a ’U-shaped’ stellar age profile

¹ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
² Institute of Space Sciences & Astronomy, University of Malta, Msida MSD 2080, Malta
³ Jeremiah Horrocks Institute, University of Lancashire, Preston PR1 2HE, UK
⁴ Department of Physics, University of Malta, Msida MSD 2080, Malta
⁵ Department of Astronomy, School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
⁶ State Key Laboratory of Dark Matter Physics, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
⁷ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁸ Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
⁹ Observatório Nacional, Rio de Janeiro, RJ 20921-400, Brazil
¹⁰ Department of Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
¹¹ Departamento de Astronomía, Universidad de La Serena, Av. Raúl Bitrán 1305, La Serena, Chile

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 November 2025
Accepted: 25 February 2026

Abstract

We leveraged reliable age and distance estimates from LAMOST-DR3 and APOGEE-DR17+AstroNN combined with Gaia data to perform a detailed analysis of the stellar age distribution in the Milky Way’s (MW) outer disc using giant stars. Selecting stars near the midplane (|z| < 0.3 kpc) on near-circular orbits (λ_c > 0.9), we analysed these independent datasets that employed different age-estimation methods. Our stringent kinematic selection criteria effectively exclude halo stars, ensuring that the observed age trends reflect genuine disc properties rather than contamination from older halo populations. Our results reveal a ’U-shaped’ stellar age profile, where a negative gradient in the inner disc transitions to a positive gradient in the outer disc region. We identify the minimum in the stellar age profile at R_min = 11.28 ± 0.58 kpc and R_min = 12.15 ± 0.62 kpc for the APOGEE-DR 17 and LAMOST-DR3 samples, respectively. Using N-body+SPH simulations, we demonstrate that R_min corresponds to the break radius in the stellar density profile (R_br), marking the edge of the Galaxy’s star-forming disc. This break arises from a sharp decline in the star formation rate, with the outer positive age gradient produced by the radial migration of stars born inside R_br. The cessation of star formation in the outer disc might be due to several mechanisms, including the dynamical influence of the bar’s outer Lindblad resonance, the onset of the Galactic warp, or thermally regulated star formation. Overall, our results support the picture that the MW has a Type II (down-bending) stellar disc with a break at R_br ≈ 11.28–12.15 kpc, where the combination of star-formation cut-off and radial migration produces the observed U-shaped age profile.