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

Abstract

The origin of the high-alpha component of the Galactic bulge remains debated, unlike the bar-driven origin of the low-alpha 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 $ \overline V_\phi(R) $ is nearly cylindrical for both low- and high-[Mg/Fe] stars across the bulge and outer bar. $\overline V_\phi(R)$ of high-[Mg/Fe] stars with [Fe/H] $ \ge -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. Orbit integrations in realistic barred Galactic potentials show that these model-independent properties correspond to a peanut bulge in the orbital density distributions for high-[Mg/Fe] stars with [Fe/H] $ \ge -0.6 $, transitioning toward a more spheroidal structure at lower metallicities. Additionally, $ \overline V_\phi$ ([Fe/H]) increases steeply as metallicity rises from about [Fe/H] $ \sim -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 $ \rm x_4 $ orbits at low metallicities to co-rotating bar-supporting $ \rm x_1 $ 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 alpha-enhanced disc, while metal-poor stars with [Fe/H] $ < -1 $ trace a more turbulent origin.