Quantifying chemical and kinematical properties of Galactic disks

TL;DR

This study analyzes chemical and kinematic data of over 119,000 giant stars to identify and characterize four distinct Galactic disk components, supporting a two-infall formation model and revealing complex formation and enrichment histories.

Contribution

It introduces a detailed classification of Galactic disks using combined chemical and kinematic data, and confirms the two-infall formation scenario with new insights into disk evolution.

Findings

Identification of four distinct disk components: hαmp, hαmr, lαmp, lαmr.

Confirmation of the two-infall formation model for the Galactic disks.

Discovery of the inverse Age-[M/H] trend in the lαmr disk.

Abstract

We aim to quantify the chemical and kinematical properties of the Galactic disks with a sample of 119,558 giant stars having abundances and 3D velocities taken or derived from the APOGEE DR17 and Gaia EDR3 catalogs. The Gaussian Mixture Model is employed to distinguish the high-$\alpha$ and low-$\alpha$ sequences along the metallicity by simutaneously using the chemical and kinematical data. Four disk components are identified and quantified that named as h$\alpha$mp, h$\alpha$mr, l$\alpha$mp, and l$\alpha$mr disks, which correspond to the features of high-$\alpha$ or low-$\alpha$, and metal-poor or metal-rich. Combined with the spatial and stellar age information, we confirm that they are well interpreted in the two-infall formation model. The first infall of turbulent gas quickly forms the hot and thick h$\alpha$mp disk with consequent thinner h$\alpha$mr and l$\alpha$mr disks. Then the second gas accretion forms a thinner and outermost l$\alpha$mp disk. We find that the inside-out and upside-down scenario does not only satisfy the overall Galactic disk formation of these two major episodes, but also presents in the formation sequence of three inner disks. Importantly, we reveal the inverse Age-[M/H] trend of the l$\alpha$mr disk, which means its younger stars are more metal-poor, indicating that the rejuvenate gas from the second accretion gradually dominates the later star formation. Meanwhile, the recently formed stars convergence to [M/H]$\sim$-0.1 dex, demonstrating a sufficiently mixture of gas from two infalls.