2016A&A...586A.112R

We study the effects of the cosmological assembly history on the chemical and dynamical properties of the discs of spiral galaxies as a function of radius. We made use of the simulated Milky Way mass, fully-cosmological discs from Ramses Disc Environment Study (RaDES). We analysed their assembly history by examining the proximity of satellites to the galactic disc, instead of their merger trees, to better gauge which satellites impact the disc. We presented stellar age and metallicity profiles, age-metallicity relation (AMR), age-velocity dispersion relation (AVR), and stellar age distribution (SAD) in several radial bins for the simulated galaxies. Assembly histories can be divided into three different stages: i) a merger dominated phase, when a large number of mergers with mass ratios of ∼1:1 take place (lasting ∼3.2±0.4Gyr on average); ii) a quieter phase, when ∼1:10 mergers take place (lasting ∼4.4±2.0Gyr); and iii) a secular phase where the few mergers that take place have mass ratios below 1:100, which do not affect the disc properties (lasting ∼5.5±2.0Gyr). The first two phases are able to kinematically heat the disc and produce a disc that is chemically mixed over its entire radial extension. Phase 2 ends with a final merger event (at time t_jump) marking the onset of important radial differences in the AMR, AVR, and SAD. Inverted AMR trends in the outer parts of discs, for stars younger than t_jump, are found as the combined effect of radial motions and star formation in satellites temporarily located in these outer parts. U-shaped stellar age profiles change to an old plateau (∼10Gyr) in the outer discs for the entire RaDES sample. This shape is a consequence of inside-out growth of the disc, radial motions of disc stars (inwards and outwards), and the accretion of old stars from satellites. We see comparable age profiles even when ignoring the influence of stellar migration due to the presence of early in situ star formation in the outer regions of the galaxy.