The Chemical Abundance Structure of the Inner Milky Way: A Signature of "Upside-Down" Disk Formation?
Abstract
We present a model for the [alpha/Fe]-[Fe/H] distribution of stars in the inner Galaxy, R=3-5 kpc, measured as a function of vertical distance |z| from the midplane by Hayden et al. (2015, 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 zh=0.8 kpc to zh=0.2 kpc over a timescale tc, remaining constant thereafter. We assume a linear-exponential star-formation history, SFR ~ te-t/tsf. With a star-formation efficiency timescale of 2 Gyr, an outflow mass-loading factor of 1.5, tsf=3 Gyr, and tc=2.5 Gyr, the model reproduces the observed locus of inner disk stars in [alpha/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 tc=1 Gyr or tsf=1 Gyr fail to match the observed MDF at high-|z| and low-|z|, respectively. The inferred scale-height evolution, with zh(t) dropping on a timescale tc ~ tsf 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-tc 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.