Simulating the formation and evolution of disc galaxies in a LambdaCDM universe

The majority of stars in the universe has formed in disc galaxies
with masses similar to that of the Milky Way. Ab-initio
cosmological hydrodynamical simulations of the formation and
evolution of galaxies in a Lambda Cold Dark Matter universe have
long suffered from serious problems in correctly modelling the
star-formation history and structure of disc galaxies. We first use
idealized semi-cosmological simulations to gain a better
understanding of processes leading to problems in disc formation
simulations. We add rotating spheres of hot gas to cosmological
dark-matter-only simulations of individual haloes and follow the
formation and evolution of galaxy discs from the cooling gas. The
initial orientation of the baryonic angular momentum with respect
to the halo has a major effect on disc formation. Despite the
coherently rotating initial conditions, the orientations of the
disc and the outer gas and the relative angle between the
components can all change by more than 90 degrees over several
billion years. Dominant discs with realistic structural and
kinematical properties form preferentially if slow cooling times
shift disc formation to later times, if the initial angular
momentum is aligned with the halo minor axis and if there is little
reorientation of the disc. We then present a new set of fully
cosmological simulations with an updated multiphase smoothed
particle hydrodynamics galaxy formation code. The update includes
improved treatment of metal-line cooling, metal production,
turbulent diffusion of metals, kinetic and thermal supernova
feedback and radiation pressure from massive young stars. We
compare the models to a variety of observations at high and low
redshifts and find good agreement for morphologies,
stellar-to-dark-matter mass ratios, star formation rates, gas
fractions and heavy element abundances. Agreement is better at
redshift z=1 than at present day as discrepancies in star formation
histories for the lowest and highest simulated galaxy masses become
apparent at late times. 18 out of 19 of our model galaxies at z=0
contain stellar discs with kinematic disc fractions up to 65 %,
higher than in any previous simulations. We finally compare our
model galaxies in detail with recent observations of the structural
evolution of stellar galactic discs and the structure of z=0 gas
discs. Stellar surface density profiles agree well with
observations at z>1, but reveal too little central growth
afterwards. This is likely connected to a lack of bars in our
simulations resulting from overly strong feedback. Discs at z=0 are
too extended by a factor \sim 2. The discs have diverse formation
histories ranging from pure inside-out growth in systems with
quiescent merger histories to continuous mass growth at all radii.
Central mass growth in our models is driven by mergers and
misaligned infall events, which leave signatures in the present day
distributions of radii and element abundances as functions of
stellar age. Gas discs agree well with observations in terms of
sizes and profile shapes, but on average have overly high
gas-to-stellar mass ratios. Our models agree well with the observed
neutral hydrogen mass-size relation. Despite significant progress,
our models continue to suffer from various problems illustrating
that we are still far away from capturing all relevant physical
processes accurately.