The formation and evolution of Milky Way sized galaxies in high-resolution cosmological zoom simulations

Simulations are playing an increasingly important role in probing the formation history of the Milky Way, including the formation of the thick/thin disc and origin of the metal distribution and chemo-dynamical relations. We introduce the Auriga project, a suite of high resolution cosmological-zoom simulations of Milky Way-sized galaxies simulated with the state-of-the-art cosmological magneto-hydrodynamical code AREPO, and present an analysis of the formation and evolution of the stellar disc(s) from early times to present  day. In particular, we show that ‘thickened discs’ are mainly driven by a bar (if present) and interactions with satellites of masses log10 (M/ Mo ) >= 10,  whereas other potential heating mechanisms such as spiral  arms, radial migration, and adiabatic heating from mid-plane  density growth are all sub-dominant. Interestingly, we find that  even in cases of violent satellite interactions the disc reforms  quickly (within a few giga years), producing a well-defined  disc-bulge system. In nearly all simulations the overall structure of the disc becomes gradually more radially extended and  vertically thinner with time, in support of the inside-out,  upside-down formation scenario, and without the presence of a thin/thick disc dichotomy. In addition, we comment on the mass distribution of mono-abundance populations and their relation to the bulge and disc components, which are readily comparable to observations from surveys such as APOGEE and Gaia.