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 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, SFR ~ te^{-t/t_sf}. With a star-formation efficiency timescale of 2 Gyr, an outflow mass-loading factor of 1.5, t_sf=3 Gyr, and t_c=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 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.

References in corpus (13)

• The Dartmouth Stellar Evolution Database
• The GALAH Survey: Scientific Motivation
• The Origin of the Galaxy Mass-Metallicity Relation and Implications for Galactic Outflows
• The nucleosynthesis of Al26 and Fe60 in solar metallicity stars extending in mass from 11 to 120 Msun: the hydrostatic and explosive contributions
• The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields
• The Initial-Final Mass Relation: Direct Constraints at the Low Mass End
• Tracing chemical evolution over the extent of the Milky Way's Disk with APOGEE Red Clump Stars
• On the Formation of Galactic Thick Disks
• Inflow, Outflow, Yields, and Stellar Population Mixing in Chemical Evolution Models
• Age velocity dispersion relations and heating histories in disc galaxies
• Equilibrium and Sudden Events in Chemical Evolution
• Chemodynamics of a Simulated Disc Galaxy: Initial Mass Functions and SNIa Progenitors
• A simple and general method for solving detailed chemical evolution with delayed production of iron and other chemical elements

Cited by in corpus (8)

• Chemical Cartography with APOGEE: Multi-element abundance ratios
• The bursty origin of the Milky Way thick disc
• A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE
• The Milky Way tomography with APOGEE: intrinsic density distribution and structure of mono-abundance populations
• The Gaia-ESO Survey: Carbon abundance in the Galactic thin and thick disks
• The innate origin of radial and vertical gradients in a simulated galaxy disc
• High- and Low-$\alpha$ Disk Stars Separate Dynamically at all Ages
• Stellar Migration and Chemical Enrichment in the Milky Way Disc: A Hybrid Model