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

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 $R_{\rm min}=11.28 \pm 0.58$ kpc and $R_{\rm min}=12.15\pm 0.62$ kpc for the APOGEE-DR17 and LAMOST-DR3 samples, respectively. Using N-body+SPH simulations, we demonstrate that $R_{\rm min}$ corresponds to the break radius in the stellar density profile ($R_{\rm br}$), 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 $R_{\rm br}$. 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 $R_{\rm br} \approx 11.28-12.15$ kpc, where the combination of star-formation cut-off and radial migration produces the observed U-shaped age profile.

Figures (11)

• Figure 1: 2D histograms representing the spatial coverage in galactocentric coordinates of the LAMOST-DR3 (left) and APOGEE-DR17 (right) samples. The two samples assume slightly different solar positions: $(X, Y, Z) = (8.2,0,0.015)$ kpc and $(8.125,0,0.021)$ kpc for LAMOST-DR3 and APOGEE-DR17, respectively.
• Figure 2: Left: Toomre diagrams showing the kinematics of the uncleaned LAMOST-DR3 (top row) and APOGEE-DR17 (bottom row) catalogues, (blue/green distribution) with the stars satisfying $\lambda_{\rm c} > 0.9$ and $|z|<0.3\hbox{$\>{\rm kpc}$}$ over-plotted with red markers. The inner and outer dashed curves represent the boundaries between the kinematically defined thin/thick discs and halo. Middle: Orbital anisotropy, $\beta$, of the cleaned sub-sample with errors determined via a bootstrapping method. The negative value of $\beta$ indicates that the population is tangentially biased. Right: Distribution of apparent Gaia$G$-band magnitude versus stellar age, $\tau_{\star}$, for the cleaned sub-samples.
• Figure 3: Top row shows the age error distribution for our cleaned LAMOST-DR3 (left) and APOGEE-DR17 (right) samples. The bottom row shows the 2D histograms of the relative age error versus $R$ for the two samples.
• Figure 4: Top panel: Number distribution of selected stars as a function of $R$ for the cleaned LAMOST-DR3 (red) and APOGEE-DR17 (blue) samples. Bottom panel: Fraction of stars retained after applying the additional cut in circularity $(\lambda_{c}>0.9)$ on stars in the mid-plane $(|z|<0.3\hbox{$\>{\rm kpc}$})$.
• Figure 5: Column-normalised 2D histograms of the stellar age distribution, $\tau_{\star}(R)$, for the LAMOST-DR3 (left) and APOGEE-DR17 (right) samples. The red and green markers show the mean and median profiles, respectively. The solid curves show the age profile resulting from the maximum-likelihood fitting (this fits to all individual sources without binning -- see text for details). In each panel, we also cross-plot the ML fit of the dataset in the opposite panel for comparison. The black marker reflects the location of the maximum-likelihood age minimum and associated uncertainty.