METALLICITY GRADIENTS IN THE MILKY WAY DISK AS OBSERVED BY THE SEGUE SURVEY

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 system. We present the radial metallicity gradient, Delta[Fe/H]/Delta R, as a function of height above the plane, vertical bar Z vertical bar, using 7010 main-sequence turnoff stars observed by the Sloan Extension for Galactic Understanding and Exploration survey. The sample consists of mostly old thin and thick disk stars, with a minimal contribution from the stellar halo, in the region 6 kpc < R < 16 kpc, 0.15 kpc < vertical bar Z vertical bar < 1.5 kpc. The data reveal that the radial metallicity gradient becomes flat at heights vertical bar Z vertical bar > 1 kpc. The median metallicity at large vertical bar Z vertical bar is consistent with the metallicities seen in outer disk open clusters, which exhibit a flat radial gradient at [Fe/H] similar to -0.5. We note that the outer disk clusters are also located at large vertical bar Z vertical bar; 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 vertical bar Z vertical bar. 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 vertical bar Z vertical bar 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 Schonrich & Binney, and we estimate our distance errors to be similar to 25%. We also show that we can properly correct for selection biases by assigning weights to our targets.