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

Technical Deep Dive: The edge of the Milky Way's star-forming disc

Overview

This work leveraged stellar age and distance estimates from LAMOST-DR3 and APOGEE-DR17 to analyze the stellar age distribution in the Milky Way's outer disc. The key findings are:

• The Milky Way has a "U-shaped" stellar age profile, with a negative age gradient in the inner disc transitioning to a positive gradient in the outer disc.
• The minimum in the age profile occurs at Rmin = 11.28 ± 0.58 kpc (APOGEE-DR17) and Rmin = 12.15 ± 0.62 kpc (LAMOST-DR3).
• N-body+SPH simulations demonstrate that R_min corresponds to the edge of the star-forming disc, where star formation drops sharply due to mechanisms like the bar's outer Lindblad resonance or the onset of the Galactic warp.
• The combination of the star formation cutoff and radial migration of stars born inside R_min produces the observed U-shaped age profile.

Data & Experimental Setup

Observational Samples

• LAMOST-DR3 sample: 66,615 kinematically cool (|z| < 0.3 kpc, λ_c > 0.9) giant stars with age estimates from isochrone fitting.
• APOGEE-DR17+AstroNN sample: 35,507 kinematically cool giant stars with age estimates from a neural network model.
• Both samples exclude halo stars based on kinematics, ensuring the observed age trends reflect the disc properties.

Simulation Models

• Three N-body+SPH models simulated with GASOLINE:
  • M1 and M2: Gaia-Sausage-Enceladus-like merger events
  • M3: Isolated disc formation

Results

Observational Age Profiles

• Both LAMOST-DR3 and APOGEE-DR17 datasets show a negative age gradient in the inner disc (R ≤ 10 kpc) and a positive age gradient in the outer disc (R ≥ 12 kpc).
• The minimum in the age profile occurs at Rmin = 11.28 ± 0.58 kpc (APOGEE-DR17) and Rmin = 12.15 ± 0.62 kpc (LAMOST-DR3).

Simulation Comparison

• In the simulation models, R_min corresponds to the edge of the star-forming disc, where there is a sharp drop in the star formation rate.
• The combination of this star formation cutoff and radial migration of stars born inside R_min produces the U-shaped age profile in the models.
• The simulation models show R_min in the 9.54 - 14.04 kpc range, consistent with the observational results.

Interpretation

• The U-shaped age profile and the associated break in the stellar density profile indicate that the Milky Way has a "Type II" (down-bending) disc structure.
• The edge of the star-forming disc (R_min) may be linked to the outer Lindblad resonance of the Galactic bar, a thermally regulated star formation threshold, or the onset of the Galactic warp.
• Future observations and detailed simulations will help clarify the root cause of the star formation cutoff in the Milky Way's outer disc.

Limitations & Uncertainties

• The observational samples have better coverage in the anti-centre direction, so azimuthal variations in the age profile could not be fully assessed.
• The simulation models do not capture the full complexity of the Milky Way's formation history.
• The physical mechanisms driving the star formation cutoff in the outer disc remain uncertain.

What Comes Next

• Continued analysis of stellar age and density profiles in the Milky Way and other disc galaxies, leveraging new data from Gaia, APOGEE, and other surveys.
• Detailed simulations exploring the role of bar dynamics, gas physics, and galactic warps in shaping the outer disc structure.
• Observational constraints on the Milky Way's bar properties and the onset of the galactic warp to link these to the observed disc structure.