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

Context . The present-day spatial and kinematic distribution of stars in the Milky Way provides key constraints on its internal dynamics and evolutionary history. However, identifying the correct tracers that highlight the mechanisms is far from straightforward. The best probes are stars that stand out in terms of kinematics, chemistry, or age compared to the underlying population to which they belong. Aims . We aimed to constrain stellar radial migration and, in particular, observationally study its effect on the disc dynamical heating. Methods . We selected Milky Way stars that are more metal-rich than the interstellar medium (ISM) at their guiding radius, the so-called local metal-rich (LMR) stars, and investigated 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. We investigated their kinematics and age distributions across the disc, and compared them to the stellar populations that have the same metallicity as the ISM. Results . Compared to locally born stars with metallicities equal to that of the ISM, LMR stars, at a given location, are always older (mean ages of up to 2 Gyr older) and with similar or slightly higher velocity dispersions. Furthermore, at a given metallicity, LMR stars are older at larger galactocentric radii, reflecting the fact that LMR stars 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 do find that the mean stellar eccentricity and mean ages are lower in the spiral arms. Conclusions . Our results support a well-established theoretical result that has not yet been formally confirmed via observations involving large datasets without modelling: churning is not significantly heating the Galactic disc. Furthermore, the age distribution of these stars rules out any significant contribution from Galactic fountains as their origin, and confirms the effect of the spiral arms on them. Although no clear signature of the Galactic bar is detected, its phase-mixed and diffuse influence – especially through interactions with spiral arms – cannot be excluded.