2025/08/12 by Pandey, Aakash, Gerhard, Ortwin
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR)
paper · doi:10.48550/arxiv.2508.09119
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 Vϕ(R) is nearly cylindrical for both low- and high-[Mg/Fe] stars across the bulge and outer bar. Vϕ(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. 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] ≥ -0.6 , transitioning toward a more spheroidal structure at lower metallicities. Additionally, Vϕ ([Fe/H]) increases steeply as metallicity rises 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 \rm x4 orbits at low metallicities to co-rotating bar-supporting \rm 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 alpha-enhanced disc, while metal-poor stars with [Fe/H] < -1 trace a more turbulent origin.