Dynamical properties of high-Mg/Fe stars in the Milky Way bar region

Abstract The origin of the high-α component of the Galactic bulge remains debated, unlike the bar-driven origin of the low-α bulge. We examine the metallicity-dependent dynamical properties of high-Mg/Fe stars in the bar region, using samples of low- and high-Mg/Fe stars from APOGEE DR17, complemented by the PIGS catalogue of Fe/H −1 stars. The mean Galactocentric rotational velocity V (R) is nearly cylindrical for both low- and high-Mg/Fe stars across the bulge and outer bar. V (R) of high-Mg/Fe stars with Fe/H ≥ −0. 6 is similar within errors to low-Mg/Fe stars in the bulge, and 10-20% lower in the outer bar. The mean radial velocity field of these stars exhibits a quadrupole pattern similar to low-Mg/Fe stars. Integrating orbits in realistic barred Galactic potentials, these model-independent properties correspond to a peanut bulge in the orbital density distributions for high-Mg/Fe stars with Fe/H ≥ −0. 6, transitioning toward a more spheroidal structure at lower metallicities. Additionally, V (Fe/H) for stars increases steeply as metallicity increases from about Fe/H ∼−1. 3, resembling the spin-up observed at larger Galactic radii. This is accompanied by a transition in the dominant orbit families, from co- and counter-rotating cloud A and x4 orbits at low metallicities to co-rotating bar-supporting x1 family tree, box, and cloud A orbits at solar metallicity. Our results strengthen the case that the bulk of the high-Mg/Fe component in the bar region evolved from an α-enhanced disc, while metal-poor stars with Fe/H −1 trace a more turbulent origin.