Chemodynamical evolution of the Milky Way disk

II. Variations with Galactic radius and height above the disk plane^⋆

¹ Leibniz-Institut fü Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: iminchev1@gmail.com
² Centre for Astrophysics & Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia
³ Max-Planck-Institut fü Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 22 January 2014
Accepted: 14 October 2014

Abstract

In the first paper of this series (Paper I) we presented a new approach for studying the chemodynamical evolution in disk galaxies, focusing on the Milky Way. While in Paper I we studied extensively the Solar vicinity, here we extend these results to different distances from the Galactic center, looking for variations of observables that can be related to on-going and future spectroscopic surveys. By separating the effects of kinematic heating and radial migration, we show that migration is much more important, even for the oldest and hottest stellar population. The distributions of stellar birth guiding radii and final guiding radii (signifying contamination from migration and heating, respectively) widen with increasing distance from the Galactic center. As a result, the slope in the age-metallicity relation flattens significantly at Galactic radii larger than solar. We predict that the metallicity distributions of (unbiased) samples at different distances from the Galactic center peak at approximately the same value, [Fe/H] ≈ −0.15 dex, and have similar metal-poor tails extending to [Fe/H] ≈ −1.3 dex. In contrast, the metal-rich tail decreases with increasing radius, thus giving rise to the expected decline of mean metallicity with radius. Similarly, the [Mg/Fe] distribution always peaks at ≈0.15 dex, but its low-end tail is lost as radius increases, while the high-end diminishes at [Mg/Fe] ≈ 0.45 dex. The radial metallicity and [Mg/Fe] gradients in our model show significant variations with height above the plane because of changes in the mixture of stellar ages. An inversion in the radial metallicity gradient is found from negative to weakly positive (at r< 10 kpc), and from positive to negative for the [Mg/Fe] gradient, with increasing distance from the disk plane. We relate this to the combined effect of (i) the predominance of young stars close to the disk plane and old stars away from it; (ii) the more concentrated older stellar component; and (iii) the flaring of mono-age disk populations. We also investigate the effect of recycled gas flows on the mean [Fe/H] and find that in the region 4 <r< 12 kpc the introduced errors are less than 0.05–0.1 dex, related to the fact that inward and outward flows mostly cancel in that radial range. We show that radial migration cannot compete with the inside-out formation of the disk, exposed by the more centrally concentrated older disk populations, and consistent with recent observations.