The Chemical Abundance Structure of the Inner Milky Way: A Signature of “Upside-down” Disk Formation

We present a model for the [α /{Fe}]{--}[{Fe}/{{H}}] distribution of stars in the inner Galaxy, 3 {kpc}< R< 5 {kpc}, measured as a function of vertical distance | z| from the midplane by Hayden et al. (H15). Motivated by an “upside-down” scenario for thick disk formation, in which the thickness of the star-forming gas layer contracts as the stellar mass of the disk grows, we combine one-zone chemical evolution with a simple prescription in which the scale-height of the stellar distribution drops linearly from {z}_h=0.8 {kpc} to {z}_h=0.2 {kpc} over a timescale t _c , remaining constant thereafter. We assume a linear-exponential star formation history, {\dot{M}}_* (t)\propto {{te}}^-t/{t_{sf}}. With a star formation efficiency timescale {τ }_* ={M}_g(t)/{\dot{M}}_* (t)=2 {Gyr}, an outflow mass-loading factor η ={\dot{M}}_{out}(t)/{\dot{M}}_* (t)=1.5, {t}_{sf}=3 {Gyr}, and {t}_c=2.5 {Gyr}, the model reproduces the observed locus of inner disk stars in [α /{Fe}]{--}[{Fe}/{{H}}] and the metallicity distribution functions (MDFs) measured by H15 at | z| =0{--}0.5 {kpc}, 0.5{--}1 {kpc}, and 1{--}2 {kpc}. Substantial changes to model parameters lead to disagreement with the H15 data; for example, models with {t}_c=1 {Gyr} or {t}_{sf}=1 {Gyr} fail to match the observed MDF at high-| z| and low-| z| , respectively. The inferred scale-height evolution, with z _h (t) dropping on a timescale {t}_c∼ {t}_{sf} at large lookback times, favors upside-down formation over dynamical heating of an initially thin stellar population as the primary mechanism regulating disk thickness. The failure of our short-t _c models suggests that any model in which thick disk formation is a discrete event will not reproduce the continuous dependence of the MDF on | z| found by H15. Our scenario for the evolution of the inner disk can be tested by future measurements of the | z| -distribution and the age-metallicity distribution at R=3{--}5 {kpc}.