Chemical Cartography With Apogee: Large-Scale Mean Metallicity Maps Of The Milky Way Disk

Michael R. Hayden
Jon A. Holtzman
Jo Bovy
Steven R. Majewski
Jennifer A. Johnson
Carlos Allende Prieto
Timothy C. Beers
Katia Cunha
Peter M. Frinchaboy
Ana E. García Pérez
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We present Galactic mean metallicity maps derived from the first year of the SDSS-III APOGEE experiment. Mean abundances in different zones of projected Galactocentric radius (0 < R < 15 kpc) at a range of heights above the plane (0 < vertical bar z vertical bar < 3 kpc), are derived from a sample of nearly 20,000 giant stars with unprecedented coverage, including stars in the Galactic mid-plane at large distances. We also split the sample into subsamples of stars with low- and high-[alpha/M] abundance ratios. We assess possible biases in deriving the mean abundances, and find that they are likely to be small except in the inner regions of the Galaxy. A negative radial metallicity gradient exists over much of the Galaxy; however, the gradient appears to flatten for R < 6 kpc, in particular near the Galactic mid-plane and for low-[alpha/M] stars. At R > 6 kpc, the gradient flattens as one moves off the plane, and is flatter at all heights for high-[alpha/M] stars than for low-[alpha/M] stars. Alternatively, these gradients can be described as vertical gradients that flatten at larger Galactocentric radius; these vertical gradients are similar for both low- and high-[alpha/M] populations. Stars with higher [alpha/M] appear to have a flatter radial gradient than stars with lower [alpha/M]. This could suggest that the metallicity gradient has grown steeper with time or, alternatively, that gradients are washed out over time by migration of stars.

Hayden, Michael R., Jon A. Holtzman, Jo Bovy, Steven R. Majewski, Jennifer A. Johnson, Carlos Allende Prieto, Timothy C. Beers et al. >Chemical Cartography with APOGEE: Large-scale Mean Metallicity Maps of the Milky Way Disk.> The Astronomical Journal, 147, no. 5 (May., 2014): 116.