The structure and evolution of the Galactic high-$α$ disc I. Chemical and age orbital cartography

Abstract

We present a comprehensive chemical and age orbital cartography of the Galactic high-$α$ disc using subgiant stars with precise ages, element abundances, and full phase-space information from the \textsl{LAMOST--Gaia} data set. Specifically, we map how average [Fe/H], [$α$/Fe], and age vary across present-day kinematic and orbital coordinates. We analyse the data in full and across mono-abundance populations to measure element abundance-orbital and age-orbital gradients across orbital actions and angular-momenta. Our results show that the high-$α$ disc exhibits clear and coherent gradients in [Fe/H], [$α$/Fe], and age with orbits; these gradients are much stronger and sharper in orbital space than in present-day kinematics, showing that orbital diagnostics recover the intrinsic disc structure of old disc populations more effectively than instantaneous kinematic coordinates. We find that older high-$α$ populations display qualitatively similar element abundance--orbital and age--orbital trends to stars in the low-$α$ disc, although the high-$α$ gradients are generally shallower. The presence of these ordered correlations indicates that the old high-$α$ disc is structured, and preserved a strong fossil record of its early assembly despite the Milky Way's subsequent accretion history. This result implies that later mergers did not fully erase the chemical-orbital and age-orbital structure imprinted during the high-$α$ disc's earliest formation epoch. All together, these findings indicate that the Galactic high-$α$ disc formed mainly through inside-out and upside-down growth.

Figures (12)

• Figure 1: Chemical and spatial distributions of the stellar sample. The left panel shows the [$\alpha/$Fe]-[Fe/H] density map for the full dataset. The dashed curve indicates the adopted boundary used to define the high-$\alpha$ population. The grey region corresponds to [Fe/H] $\leq$ -1, which is not considered in the high-$\alpha$ selection. The middle and right panels show the spatial distribution ($x$--$z$) and the age-metallicity relation, respectively, for the stars classified as high-$\alpha$. Median uncertainties are shown in the corners of the panels. In the right panel, the ages of subgiant stars go above 14 Gyr due to the flat prior adopted in Xiang2022. However, most of the high-$\alpha$ disc stars have $\tau<14$ Gyr, and since this work focuses on modelling the median value across orbital properties, this tail of older stars do not affect our results.
• Figure 2: Kinematic and orbital distributions of the high-$\alpha$ population. The top row shows maps in the $R$-$|z|$ plane, while the bottom row displays the corresponding distributions in the $R_g$-$z_{\max}$ plane. From left to right, panels are colour-coded by median of [Fe/H], [$\alpha$/Fe], and stellar age. The high-$\alpha$ disc shows clear element abundance and age trends as a function of kinematics and orbits, which resemble that of stars in the low-$\alpha$ disc (see Appendix \ref{['app_lowalpha']}). However, these trends are much stronger as a function of orbital properties.
• Figure 3: Distribution of high-$\alpha$ stars in action space. The panels show the median values of [Fe/H] (top row), [$\alpha$/Fe] (middle row), and stellar age (bottom row) across different projections of the actions $(J_R, J_z, J_\phi)$. Colors indicate the mean chemical abundance or age within each bin, highlighting the element abundance--orbital and age--orbital structure of the high-$\alpha$ disc population.
• Figure 4: Distribution of high-$\alpha$ stars in angular momentum space. The panels show the median values of [Fe/H] (top row), [$\alpha,$/Fe] (middle row), and stellar age (bottom row) across different projections of the angular momentum components $(L_x, L_y, L_z)$. Colors indicate the median chemical abundance or median age within each bin, highlighting the element abundance--orbital and age--orbital structure of the high-$\alpha$ disc population.
• Figure 5: Correlations between element abundances, stellar ages, and actions for high-$\alpha$ disc stars. The top row shows the relation between [Fe/H] and the actions $J_R$, $J_z$, $J_\phi$, and $J_\perp$, while the middle row presents the corresponding relations for [$\alpha$/Fe]. The bottom row displays the dependence of stellar age $\tau$ on the same actions. Marker colors indicate the complementary abundance ([$\alpha$/Fe] in the top row, [Fe/H] in the middle row) and the [Fe/H] ratio in the bottom row, while marker sizes scale with the number of stars in each mono-abundance bin. Typical uncertainties of the median values, estimated as $\sigma/\sqrt{n}$ for each mono-abundance bin, are shown as representative error bars in the lower-right corner of each panel. See the text in Section\ref{['sec_maps']} for details.