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

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.

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

The study on the formation and evolution of the Milky Way's disk system by Lee et al. utilizes a sample of G-type dwarfs from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. The primary focus is the kinematic analysis and [α/Fe] ratios derived from low-resolution spectra, emphasizing the distinction between thin- and thick-disk populations.

[α/Fe] Segregation and Kinematic Trends

The authors employed [α/Fe] chemical abundances to differentiate between the thin and thick disks of the Milky Way, as opposed to the conventional kinematic or spatial classifications. This approach revealed distinct kinematic features between the two populations. For the thin-disk stars, a negative gradient of rotational velocity with metallicity was observed, ranging from -20 to -30 km s$^{-1}$ dex$^{-1}$. Conversely, the thick-disk stars exhibited a strong positive gradient of +40 to +50 km s$^{-1}$ dex$^{-1}$.

Additionally, the study found negligible gradients of rotational velocity with Galactocentric distance for the thin-disk stars and a small but noticeable gradient for the thick-disk stars. Both the thin- and thick-disk populations showed a decline in rotational velocity with increasing distance from the Galactic plane, approximately -9.0 km s$^{-1}$ kpc$^{-1}$.

Orbital Eccentricity Analysis

Within the thin-disk population, orbital eccentricity demonstrated constancy with respect to metallicity, indicating a tight distribution centered around e ≈ 0.14. In contrast, the thick disk showed increasing eccentricity as metallicity decreased, suggesting greater dynamical mixing over time. Eccentricity for both populations increased with distance from the Galactic plane, displaying similar trending behaviors.

Implications and Interpretations

The overarching implications point towards significant contributions from radial migration in shaping the thin-disk structure. The gradients correlate well with predictions from models of radial mixing and star formation dynamics within disk galaxies. For the thick disk, the findings challenge some of the existing formation hypotheses, indicating that gas-rich mergers or minor merger-induced heating might have played pivotal roles in shaping this component.

The orbital eccentricity distributions and velocity trends further provide constraints for potential disk evolution scenarios. The observed extended tails of higher eccentricities suggest that purely accretion-based models are less viable for the Milky Way's thick disk.

The findings advocate for continuously improved simulations and theoretical models to better elucidate the evolutionary pathways of the Milky Way’s disk components. As observational techniques advance and more comprehensive data become available, they will undoubtedly refine our understanding of galactic formation mechanisms and the role of distinct stellar populations in galactic dynamics.

Future Prospects

Further exploration into improved simulation models, especially those considering initial conditions and star formation histories, could offer more definitive insights into the role of secular processes compared to violent origins of the thick disk. These models should aim to account for the complexities and overlapping influences of different formation scenarios, as highlighted by the discrepancies and confirmations in this study.

In conclusion, the examination of [α/Fe] ratios and kinematics in this comprehensive analysis provides an essential framework for understanding the dynamic structure and evolution of the Milky Way, paving the way for deeper exploration into galactic archeology.