Using G dwarfs from the Sloan Extension for Galactic Understanding and Exploration ( SEGUE ) survey , we have determined a vertical metallicity gradient over a large volume of the Milky Way ’ s disk , and examined how this gradient varies for different [ \alpha /Fe ] subsamples .
This sample contains over 40,000 stars with low-resolution spectroscopy over 144 lines of sight .
We employ the SEGUE Stellar Parameter Pipeline ( SSPP ) to obtain estimates of effective temperature , surface gravity , [ Fe/H ] , and [ \alpha /Fe ] for each star and extract multiple volume-complete subsamples of approximately 1000 stars each .
Based on the survey ’ s consistent target-selection algorithm , we adjust each subsample to determine an unbiased picture of the disk in [ Fe/H ] and [ \alpha /Fe ] ; consequently , each individual star represents the properties of many .
The SEGUE sample allows us to constrain the vertical metallicity gradient for a large number of stars over a significant volume of the disk , between \sim 0.3 and 1.6 kpc from the Galactic plane , and examine the in situ structure , in contrast to previous analyses which are more limited in scope .
This work does not pre-suppose a disk structure , whether composed of a single complex population or a distinct thin and thick disk component .
The metallicity gradient is - 0.243 ^ { +0.039 } _ { -0.053 } dex kpc ^ { -1 } for the sample as a whole , which we compare to various literature results .
Each [ \alpha /Fe ] subsample dominates at a different range of heights above the plane of the Galaxy , which is exhibited in the gradient found in the sample as a whole .
Stars over a limited range in [ \alpha /Fe ] show little change in median [ Fe/H ] with height .
If we associate [ \alpha /Fe ] with age , our consistent vertical metallicity gradients with [ \alpha /Fe ] suggest that stars formed in different epochs exhibit comparable vertical structure , implying similar star-formation processes and evolution .