MaGICC thick disc - I. Comparing a simulated disc formed with stellar feedback to the Milky Way

We analyse the structure and chemical enrichment of a Milky Way-like galaxy with a stellar mass of 2 x 10(10) M-circle dot, formed in a cosmological hydrodynamical simulation. It is disc dominated with a flat rotation curve, and has a disc scalelength similar to the Milky Way's, but a velocity dispersion that is similar to 50 per cent higher. Examining stars in narrow [Fe/H] and [alpha/Fe] abundance ranges, we find remarkable qualitative agreement between this simulation and observations. (a) The old stars lie in a thickened distribution with a short scalelength, while the young stars form a thinner disc, with scalelengths decreasing, as [Fe/H] increases. (b) Consequently, there is a distinct outward metallicity gradient. (c) Mono-abundance populations exist with a continuous distribution of scaleheights (from thin to thick). However, the simulated galaxy has a distinct and substantive very thick disc (h(z) similar to 1.5 kpc), not seen in the Milky Way. The broad agreement between simulations and observations allows us to test the validity of observational proxies used in the literature: we find in the simulation that mono-abundance populations are good proxies for single age populations (<1Gyr) for most abundances.