The diverse formation histories of simulated disc galaxies

We analyse 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 I > cold dark matter 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 a parts per thousand yen 1.5. At z a parts per thousand currency sign 1, there is too much growth in size and too little growth in central mass, possibly due to the underabundance of bars. The discrepancies may partly be caused by differences between the star formation histories of the simulations and those assumed for observations.