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.
