MaGICC Thick Disk I: Comparing a Simulated Disk Formed with Stellar Feedback to the Milky Way

We make a detailed comparison of a recent simulation of the formation of a Milky Way-like galaxy in a cosmological context with recent observations of how the structural and kinematic properties of the Galactic disk vary with stellar abundance. The simulations are part of the Making Galaxies in a Cosmological Context (MaGICC) project, a series of fully cosmological smoothed particle hydrodynamics (SPH) simulations. The simulated galaxy has a stellar mass of 2e10 Msun, and a halo mass of 7e11 Msun, and resembles the Milky Way: it is disk-dominated with a flat rotation curve, and the scale length and height of the disk are similar to the Milky Way's. The simulated stellar disk is, however, kinematically ~50% hotter than the Milky Way. Examining stars in narrow [Fe/H] and [α/Fe] abundance ranges, we find remarkable qualitative agreement between the simulations and observations: a) The old stars lie in a thickened distribution with a short scale length while the young stars form a thin distribution with a wide range of scale lengths that depend on their [Fe/H] metallicity. b) There is an outward metallicity gradient. c) Mono-abundance populations exist with a continuous distribution of scale heights (from thin to thick) and of contributions to the stellar surface disk mass. The simulation differs in some details from the observed Galactic disk structure: a) The α-enrichment peaks at [O/Fe]~0.25 rather than [α/Fe]~0.5. b) In the simulations, there is a distinct very thick, disk-like component, a remnant of the early, turbulent formation phase of the simulated disk, unlike the Milky Way. This broad agreement between simulations and observations allows us to test the validity of observational proxies used in the literature: we find in the MaGICC simulations that mono-abundance populations are good proxies for single age populations (<1 Gyr) for most abundances. [Abridged]