Formation and Evolution of the Disk System of the Milky Way: [alpha/Fe] Ratios and Kinematics of the SEGUE G-Dwarf Sample

TL;DR

This study uses [alpha/Fe] ratios and kinematic data from SEGUE G-dwarfs to analyze the formation and evolution of the Milky Way's disk, revealing distinct properties of thin and thick disks and suggesting radial migration's role.

Contribution

It provides a detailed analysis of the kinematic and chemical properties of the Milky Way's disk components using low-resolution spectra, highlighting differences and the potential influence of radial migration.

Findings

Strong opposite gradients of orbital velocity with metallicity in thin and thick disks.

Eccentricity varies with metallicity in the thick disk but not in the thin disk.

Radial migration likely influences thin-disk evolution more than thick-disk.

Abstract

We employ measurements of the [alpha/Fe] ratio derived from low-resolution (R~2000) spectra of 17,277 G-type dwarfs from the SEGUE survey to separate them into likely thin- and thick-disk subsamples. Both subsamples exhibit strong gradients of orbital rotational velocity with metallicity, of opposite signs, -20 to -30 km/s/dex for the thin-disk and +40 to +50 km/s/dex for the thick-disk population. The rotational velocity is uncorrelated with Galactocentric distance for the thin-disk subsample, and exhibits a small trend for the thick-disk subsample. The rotational velocity decreases with distance from the plane for both disk components, with similar slopes (-9.0 {\pm} 1.0 km/s/kpc). Thick-disk stars exhibit a strong trend of orbital eccentricity with metallicity (about -0.2/dex), while the eccentricity does not change with metallicity for the thin-disk subsample. The eccentricity is almost independent of Galactocentric radius for the thin-disk population, while a marginal gradient of the eccentricity with radius exists for the thick-disk population. Both subsamples possess similar positive gradients of eccentricity with distance from the Galactic plane. The shapes of the eccentricity distributions for the thin- and thick-disk populations are independent of distance from the plane, and include no significant numbers of stars with eccentricity above 0.6. Among several contemporary models of disk evolution we consider, radial migration appears to have played an important role in the evolution of the thin-disk population, but possibly less so for the thick disk, relative to the gas-rich merger or disk heating scenarios. We emphasize that more physically realistic models and simulations need to be constructed in order to carry out the detailed quantitative comparisons that our new data enable.