Hierarchical formation of bulgeless galaxies: Why outflows have low
  angular momentum
  Using high resolution, fully cosmological smoothed particle hydro-dynamical
simulations of dwarf galaxies in a Lambda cold dark matter Universe, we show
how baryons attain a final angular momentum distribution which allows pure disc
galaxies to form. Blowing out substantial amounts of gas through supernovae and
stellar winds, which is well supported observationally, is a key ingredient in
forming bulgeless discs. We outline why galactic outflows preferentially remove
low angular momentum material, and show that this is a natural result when
structure forms in a cold dark matter cosmology. The driving factors are a) the
mean angular momentum of accreted material increases with time, b) lower
potentials at early times, c) the existence of an extended reservoir of high
angular momentum gas which is not within star forming regions, meaning that
only gas from the inner region (low angular momentum gas) is expelled and d)
the tendency for outflows to follow the path of least resistance which is
perpendicular to the disc. We also show that outflows are enhanced during
mergers, thus expelling much of the gas which has lost its angular momentum
during these events, and preventing the formation of "classical", merger driven
bulges in low mass systems. Stars formed prior to such mergers form a diffuse,
extended stellar halo component.
