The distribution of [$\alpha$/Fe] in the Milky Way disc

Abstract

Using a sample of red giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16, we infer the conditional distribution $p([\alpha /\text{Fe}]|[\text{Fe/H}])$ in the Milky Way disk for the $\alpha$-elements Mg, O, Si, S, and Ca. In each bin of [Fe/H] and Galactocentric radius $R$, we model $p([\alpha /\text{Fe}])$ as a sum of two Gaussians, representing "low-$\alpha$" and "high-$\alpha$" populations with scale heights $z_1=0.45\text{kpc}$ and $z_2=0.95\text{kpc}$, respectively. By accounting for age-dependent and $z$-dependent selection effects in APOGEE, we infer the [$\alpha$/Fe] distributions that would be found for a fair sample of long-lived stars covering all $z$. Near the Solar circle, this distribution is bimodal at sub-solar [Fe/H], with the low-$\alpha$ and high-$\alpha$ peaks clearly separated by a minimum at intermediate [$\alpha$/Fe]. In agreement with previous results, we find that the high-$\alpha$ population is more prominent at smaller $R$, lower [Fe/H], and larger $|z|$, and that the sequence separation is smaller for Si and Ca than for Mg, O, and S. We find significant intrinsic scatter in [$\alpha$/Fe] at fixed [Fe/H] for both the low-$\alpha$ and high-$\alpha$ populations, typically $\sim 0.04$-dex. The means, dispersions, and relative amplitudes of this two-Gaussian description, and the dependence of these parameters on $R$, [Fe/H], and $\alpha$-element, provide a quantitative target for chemical evolution models and a test for hydrodynamic simulations of disk galaxy formation. We argue that explaining the observed bimodality will probably require one or more sharp transitions in the disk's gas accretion, star formation, or outflow history in addition to radial mixing of stellar populations.

Accepted for publication in MNRAS

References in corpus (16)

• New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star forming dwarf galaxies
• APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South
• Target Selection for the SDSS-IV APOGEE-2 Survey
• Tracing chemical evolution over the extent of the Milky Way's Disk with APOGEE Red Clump Stars
• The age-metallicity structure of the Milky Way disk
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• Origin of chemically distinct discs in the Auriga cosmological simulations
• Abundance gradients in the Milky Way for alpha elements, Iron peak elements, Barium, Lanthanum and Europium
• How to quench a galaxy
• The imprint of clump formation at high redshift. I. A disc alpha-abundance dichotomy
• Age dating of an early Milky Way merger via asteroseismology of the naked-eye star $\nu$ Indi
• Stellar migrations and metal flows -- Chemical evolution of the thin disc of a simulated Milky Way analogous galaxy
• From 'bathtub' galaxy evolution models to metallicity gradients
• Chemical evolution of the Milky Way: constraints on the formation of the thick and thin discs
• Age-chemical abundance structure of the Galaxy I: Evidence for a late accretion event in the outer disc at z ~ 0.6
• The RAdial Velocity Experiment (RAVE): Parameterisation of RAVE spectra based on convolutional neural networks

Cited by in corpus (19)

• A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE
• How Many Elements Matter?
• On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios
• Chemical Cartography with APOGEE: Mapping Disk Populations with a Two-Process Model and Residual Abundances
• [Mg/Fe] ratios in the solar neighbourhood: stellar yields and chemical evolution scenarios
• The Flattening Metallicity Gradient in the Milky Way's Thin Disk
• Empirical Constraints on the Nucleosynthesis of Nitrogen
• Residual Abundances in GALAH DR3: Implications for Nucleosynthesis and Identification of Unique Stellar Populations
• Universal properties of the high- and low-α disk: small intrinsic abundance scatter and migrating stars
• Galactic Chemical Evolution Models Favor an Extended Type Ia Supernova Delay-Time Distribution
• Disc dichotomy signature in the vertical distribution of [Mg/Fe] and the delayed gas infall scenario
• Untangling the Sources of Abundance Dispersion in Low-Metallicity Stars
• On the variation in stellar $\alpha$-enhancements of star-forming galaxies in the EAGLE simulation
• Stellar Population Astrophysics (SPA) with the TNG. alpha-elements, lithium, sodium and aluminum in 16 open clusters
• A Galactic Self-Portrait: Density Structure and Integrated Properties of the Milky Way Disk
• Dissecting stellar populations with manifold learning I. Validation of the method on a synthetic Milky Way-like galaxy
• Quantifying chemical and kinematical properties of Galactic disks
• Primordial helium-3 redux: The helium isotope ratio of the Orion nebula
• Revisiting the Perseus Cluster III: Role of Aspherical Explosions on its Chemical Composition and Extension to Metal-Poor Stars and Galaxies