Numerical Simulation of a possible origin of the positive radial metallicity gradient of the thick disk

Abstract

We analyze the radial and vertical metallicity and [alpha/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemodynamical galaxy evolution code, GCD+. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity ([alpha/Fe]) gradient becomes more positive (negative) with increasing height, changing sign around 1.5 kpc above the plane. At the largest vertical height (2 < |z| < 3 kpc), our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where the younger stars have higher metallicity and lower velocity dispersion. Due to the age-velocity dispersion relation, a greater fraction of younger stars reach |z| > 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e. flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al. (2013), we argue that the observed (small) positive radial metallicity gradient at large heights of the Milky Way disk stars can be explained by the flaring of the younger thick and/or thin disk stars.

10 pages, 4 figures. Revised version. Accepted for publication in RAA

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• Ages and masses of million Galactic disk main sequence turn-off and sub-giant stars from the LAMOST Galactic spectroscopic surveys
• A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE
• Aurigaia: mock Gaia DR2 stellar catalogues from the Auriga cosmological simulations
• Chasing the impact of the Gaia-Sausage-Enceladus merger on the formation of the Milky Way thick disc
• Impacts of a Flaring Star-forming Disc and Stellar Radial Mixing on the Vertical Metallicity Gradient
• Origin of the Metallicity Distribution in the Thick Disc
• Yule-Simpson's paradox in Galactic Archaeology
• Milky Way's Thick and Thin disk: Is there distinct thick disk?
• The vertical metallicity gradients of mono-age stellar populations in the Milky Way with the RAVE and Gaia data
• Unravelling stellar populations in the Andromeda Galaxy
• Impacts of Radial Mixing on the Galactic Thick and Thin Disks