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 ${\overline{V}}_{\varphi }(R)$ is nearly cylindrical for both low- and high-[Mg/Fe] stars across the bulge and outer bar. ${\overline{V}}_{\varphi }(R)$ 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, ${\overline{V}}_{\varphi }$ ([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 ${\mathrm{x}}_4$ orbits at low metallicities to co-rotating bar-supporting ${\mathrm{x}}_1$ 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.

18 pages, 16 PDF figures; accepted in MNRAS; originally submitted to MNRAS in February 2025

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