Formation and Evolution of the Disk System of the Milky Way: [alpha/Fe] Ratios and Kinematics of the SEGUE G-Dwarf Sample
We employ measurements of the [{alpha}/Fe] ratio fromlow-resolution (R {approx} 2000) spectra of 17,500 G-type dwarfs included in SDSS Data Release 8, selected using simple and well-understood selection criteria, to separate them into likely thin- and thick-disk subsamples. This classification, based on chemistry, is strongly motivated by the bi-modal distribution of stars in the [{alpha}/Fe] vs. [Fe/H] diagram. The resulting subsamples allow, for the first time, investigations of the kinematic behavior of thin- and thick-disk stars as a function of metallicity and position up to distances of 3 kpc from the Galactic plane. Both subsamples exhibit strong gradients of orbital rotational velocity with metallicity, but with opposite signs (-20 to -30 km s{sup -1} dex{sup -1} for the thin-disk population, and +40 to +50 km s{sup -1} dex{sup -1} for the thick-disk population). We find that the rotational velocity decreases with the distance from the plane for both disk components, with similar slopes (10 km s{sup -1} kpc{sup -1}), and a nearly constant difference in the mean rotational velocity of about 30 km s{sup -1}. The mean rotational velocity is uncorrelated with Galactocentric distance for the thin-disk subsample, and exhibits only a marginally significant correlation for the thick-disk subsample. Thick-disk stars exhibit a very strong trend of orbital eccentricity with metallicity (-0.2 dex{sup -1}), 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 stars, while a marginal gradient of the eccentricity with distance exists for the thick-disk population. Both subsamples possess similar trends of increasing eccentricity with distance from the Galactic plane, with a constant difference of about 0.1. The shapes of the overall distributions of orbital eccentricity for the thin- and thick-disk populations are quite different from one another, independent of distance from the plane; neither subsample has significant numbers of stars with eccentricity above 0.6. These observational results provide strong new constraints on models for the formation and evolution of the Milky Way's disk system. For example, the observed dependence of the mean rotational velocity on metallicity for thin-disk stars is inconsistent with predictions from classical local chemical evolution models. We also consider the predictions of several contemporary models of disk evolution, such as radial migration, gas-rich mergers, disk heating, and pure accretion models. We find that radial migration appears to have played an important role in the evolution of the thin-disk population, but possibly less so, relative to the gas-rich merger or disk heating scenarios, for the thick disk. Pure accretion models appear to be ruled out by the observed distribution of eccentricities for thick-disk stars. We emphasize that more physically realistic models, and simulations that probe a greater range of disk formation scenarios, need to be constructed in order to carry out the detailed quantitative comparisons that our new data enable.
- Research Organization:
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC02-07CH11359
- OSTI ID:
- 1012421
- Report Number(s):
- FERMILAB-PUB-11-186-AE-CD; arXiv eprint number arXiv:1104.3114; TRN: US201109%%843
- Journal Information:
- Submitted to Astrophys.J., Journal Name: Submitted to Astrophys.J.
- Country of Publication:
- United States
- Language:
- English
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