Judy Y. Cheng, Constance M. Rockosi, Heather L. Morrison, Ralph A. Schönrich, Young Sun Lee, Timothy C. Beers, Dmitry Bizyaev, Kaike Pan, Donald P. Schneider
The observed radial and vertical metallicity distribution of old stars in the
Milky Way disk provides a powerful constraint on the chemical enrichment and
dynamical history of the disk. We present the radial metallicity gradient,
\Delta[Fe/H]/\Delta R, as a function of height above the plane, |Z|, using 7010
main sequence turnoff stars observed by the Sloan Extension for Galactic
Understanding and Exploration (SEGUE) survey. The sample consists of mostly old
thin and thick disk stars, with a minimal contribution from the stellar halo,
in the region 6 < R < 16 kpc, 0.15 < |Z| < 1.5 kpc. The data reveal that the
radial metallicity gradient becomes flat at heights |Z| > 1 kpc. The median
metallicity at large |Z| is consistent with the metallicities seen in outer
disk open clusters, which exhibit a flat radial gradient at [Fe/H] ~ -0.5. We
note that the outer disk clusters are also located at large |Z|; because the
flat gradient extends to small R for our sample, there is some ambiguity in
whether the observed trends for clusters are due to a change in R or |Z|. We
therefore stress the importance of considering both the radial and vertical
directions when measuring spatial abundance trends in the disk. The flattening
of the gradient at high |Z| also has implications on thick disk formation
scenarios, which predict different metallicity patterns in the thick disk. A
flat gradient, such as we observe, is predicted by a turbulent disk at high
redshift, but may also be consistent with radial migration, as long as mixing
is strong. We test our analysis methods using a mock catalog based on the model
of Sch\"onrich & Binney, and we estimate our distance errors to be ~25%. We
also show that we can properly correct for selection biases by assigning
weights to our targets.
View original:
http://arxiv.org/abs/1110.5933
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