The diverse formation histories of simulated disc galaxies

We analyze the formation histories of 19 galaxies from cosmological smoothed particle hydrodynamics zoom-in resimulations. We construct mock three-colour images and show that the models reproduce observed trends in the evolution of galaxy colours and morphologies. However, only a small fraction of galaxies contains bars. Many galaxies go through phases of central mass growth by in-situ star formation driven by gas-rich mergers or misaligned gas infall. These events lead to accretion of low-angular momentum gas to the centres and leave imprints on the distributions of z=0 stellar circularities, radii and metallicities as functions of age. Observations of the evolution of structural properties of samples of disc galaxies at z=2.5-0.0 infer continuous mass assembly at all radii. Our simulations can only explain this if there is a significant contribution from mergers or misaligned infall, as expected in a LambdaCDM universe. Quiescent merger histories lead to high kinematic disc fractions and inside-out growth, but show little central growth after the last `destructive' merger at z>1.5. For sufficiently strong feedback, as assumed in our models, a moderate amount of merging does not seem to be a problem for the z=0 disc galaxy population, but may rather be a requirement. The average profiles of simulated disc galaxies agree with observations at z>=1.5. At z<=1, there is too much growth in size and too little growth in central mass, possibly due to the under-abundance of bars. The discrepancies may partly be caused by differences between the star formation histories of the simulations and those assumed for observations.