Papers › The age-metallicity structure of the Milky Way disk
The age-metallicity structure of the Milky Way disk
J. Ted Mackereth, Jo Bovy, Ricardo P. Schiavon, Gail Zasowski, Katia Cunha, Peter M. Frinchaboy, Ana E. Garcia Perez, Michael R. Hayden, Jon Holtzman, Steven R. Majewski, Szabolcs Meszaros, David L. Nidever, Marc Pinsonneault, Matthew D. Shetrone
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The measurement of the structure of stellar populations in the Milky Way disk places fundamental constraints on models of galaxy formation and evolution. Previously, the disk's structure has been studied in terms of populations defined geometrically and/or chemically, but a decomposition based on stellar ages provides a more direct connection to the history of the disk, and stronger constraint on theory. Here, we use positions, abundances and ages for 31,244 red giant branch stars from the SDSS-APOGEE survey, spanning 3 < R_(gc) < 15 kpc, to dissect the disk into mono-age and mono-[Fe/H] populations at low and high [α/Fe]. For each population, with Δage < 2 Gyr and Δ[Fe/H] < 0.1 dex, we measure the structure and surface-mass density contribution. We find that low [α/Fe] mono-age populations are fit well by a broken exponential, which increases to a peak radius and decreases thereafter. We show that this profile becomes broader with age, interpreted here as a new signal of disk heating and radial migration. High [α/Fe] populations are well fit as single exponentials within the radial range considered, with an average scale length of 1.9±0.1 kpc. We find that the relative contribution of high to low [α/Fe] populations at R₀ is f_Σ = 18% ±5%; high [α/Fe] contributes most of the mass at old ages, and low [α/Fe] at young ages. The low and high [α/Fe] populations overlap in age at intermediate [Fe/H], although both contribute mass at R₀ across the full range of [Fe/H]. The mass weighted scale height h_Z distribution is a smoothly declining exponential function. High [α/Fe] populations are thicker than low [α/Fe], and the average h_Z increases steadily with age, between 200 and 600 pc.
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