Multicomponent and Variable Velocity Galactic Outflow in Cosmological
  Hydrodynamic Simulations
  We develop a new ``Multicomponent and Variable Velocity'' (MVV) galactic
outflow model for cosmological smoothed particle hydrodynamic (SPH)
simulations. The MVV wind model reflects the fact that the wind material can
arise from different phases in the interstellar medium (ISM), and the
mass-loading factor in the MVV model is a function of galaxy stellar mass. We
find that the simulation with the MVV outflow has the following
characteristics: (i) the intergalactic medium (IGM) is hardly heated up, and
the mean IGM temperature is almost the same as in the no-wind run; (ii) it has
lower cosmic star formation rates (SFRs) compared to the no-wind run, but
higher SFRs than the constant velocity wind run; (iii) it roughly agrees with
the observed IGM metallicity, and roughly follows the observed evolution of
Omega(Civ); (iv) the lower mass galaxies have larger mass-loading factors, and
the low-mass end of galaxy stellar mass function is flatter than in the
previous simulations. Therefore, the MVV outflow model mildly alleviates the
problem of too steep galaxy stellar mass function seen in the previous SPH
simulations. In summary, the new MVV outflow model shows reasonable agreement
with observations,
  and gives better results than the constant velocity wind model.
