First large scale spatial and velocity patterns of local metal-rich stars in the Milky Way

TL;DR

This work tests the impact of radial migration (churning) on the Milky Way's disc by analyzing Local Metal-Rich (LMR) stars identified from Gaia photometric metallicities across a wide disc using Gaia DR3 kinematics. It finds that LMR stars are systematically older than locally born stars with matching ISM metallicity, yet their velocity dispersions are only marginally higher, supporting minimal disc heating by churning. At a fixed metallicity excess relative to the ISM, LMRs become older with increasing radius, consistent with time-dependent migration; above the ISM metallicity, LMRs show flat dispersions with metallicity while ages rise, indicating migration over Gyr timescales without strong heating. Spiral arms locally modulate eccentricities and ages without creating strong density enhancements in LMRs, while a clear bar signature remains elusive; these results reinforce radial migration as a key driver of disc evolution and offer observational constraints for future abundance- and birth-radius–based modelling.

Abstract

(Abridged) The present-day spatial and kinematic distribution of stars in the Milky Way provides key constraints on its internal dynamics and evolutionary history. We select stars that are more metal-rich than the interstellar medium (ISM) at their guiding radius (the so-called Local Metal-Rich stars, LMR) and investigate their chemo-kinematics. Until recently, existing catalogues did not contain such targets in large quantities, but one can now select many millions of them by using Gaia photometric metallicities. Once selected, we investigate their kinematics and age distributions across the disc, and compare them to the stellar populations having the metallicity of the ISM. Compared to locally born stars with [M/H]=[M/H]_ISM, we find that LMR stars, at a given location, are always older (mean age up to 2 Gyr older) and with velocity dispersions similar or slightly higher. Furthermore, at a given [M/H], LMR stars are older at larger galactocentric radiii, reflecting the fact that they need time to migrate. Finally, whereas we do not find any correlation between the location of the spiral arms and the spatial density of LMR stars, we find that the mean stellar eccentricity and mean ages show smaller values where the spiral arms are. Our results confirm a well established theoretical result that has not yet been formally confirmed via observations on large datasets without modelling: churning is not significantly heating the Galactic disc. Furthermore, the age distribution of these stars rule-out any significant contribution from Galactic fountains as their origin, and confirm the effect of the spiral arms on them.

Figures (10)

• Figure 1: Top: G-magnitude distributions (left) and $T_{\rm eff}$ distributions of the total XGBoost sample cross-matched with AspGap (in grey), and of the sub-sample for which differences in metallicity are smaller than 0.1 dex (in red), see Sect. \ref{['sec:Data_selection']}. Middle: comparison between the XGBoost and the AspGap metallicities, colour-coded by $T_{\rm eff}$. The plot on the right zooms-in on the high-metallicity regime. Bottom: Same as the middle plots, colour-coded by G magnitude. Black contour-lines represent 33, 66, 90 and 99 per cent of the sample. The red solid line represents the 1:1 relation, whereas the dashed ones are shifted by $\pm0.1$ dex from identity.
• Figure 2: Comparison of XGboost metallicities with GALAH DR4 iron abundances. All of the panels include the selection described in Sect. \ref{['subsec:ruwe_varpi_deltamh_selection']} as well the flag_sp==0 selection from GALAH. The colour-code is XGboost $T_{\rm eff}$, and the numbers within each plot indicate the mean offset and dispersion of the difference. The top plot considers all of the stars with $\rm{[M/H]}>-1$, whereas the bottom plot considers only the local metal-rich stars, i.e. stars with metallicities above the ISM's one at their guiding radius.
• Figure 3: Spatial distribution of stars with $\rm{[M/H]}>-0.25$ in face-on view (X-Y, panels 1 and 3) and edge-on view (R-Z, panels 2 and 4) after the selections described in Sect. \ref{['sec:Data_selection']}. The position of the Sun is indicated by a red '+' sign, at $R=8.249\,{\rm kpc}$. The two plots on the left are colour-coded by number of stars whereas the ones on the right by average metallicity. Iso-contour lines containing 33, 66, 90 and 99 per cent of the distribution are plotted inside each panel.
• Figure 4: Derived age-velocity dispersions for three different 2-kpc wide annuli (Sun being located at $R_{\rm \odot}=8.249\,{\rm kpc}$). The width of the lines corresponds to the statistical uncertainty on the dispersion ($\sigma/\sqrt{2(n-1)}$, with $n$ the number of stars in a considered bin). Dashed lines are functions in the form of $\sigma_V=\beta \cdot \tau^k$, with adopted values for $\sigma_R, \sigma_\phi, \sigma_Z$ of $\beta=$ [27,18,10] and for $k=$ [0.31,0.34,0.47]. Only stars up to 1 kpc from the plane are considered.
• Figure 5: Velocity dispersions (radial, azimuthal and vertical in the first, second and third plots, respectively) and average age (fourth plot) as a function of metallicity, for different galactocentric guiding radii $R_g$, as annotated by the legend. The 'X' symbols are located at the ISM's metallicity at the considered position, assuming the metallicity gradient derived from classical Cepheids and open clusters from daSilva23. The thickness of the line corresponds to the uncertainty on the measurement. The step in metallicity to compute the trends is 0.1 dex.