  We present the results of a numerical study on the effects of metal
enrichment and metal cooling on galaxy formation and cosmic star formation (SF)
history using cosmological hydrodynamic simulations. We find following
differences in the simulation with metal cooling when compared to the run
without it: (1) the cosmic star formation rate (SFR) is enhanced by about 50 &
20% at z=1 & 3, respectively; (2) the gas mass fraction in galaxies is lower;
(3) the total baryonic mass function (gas + star) at z=3 does not differ
significantly, but shows an increase in the number of relatively massive
galaxies at z=1; (4) the baryonic mass fraction of intergalactic medium (IGM)
is reduced at z<3 due to more efficient cooling and gas accretion onto
galaxies. Our results suggest that the metal cooling enhances the galaxy growth
by two different mechanisms: (1) increase of SF efficiency in the local
interstellar medium (ISM), and (2) increase of IGM accretion onto galaxies. The
former process is effective throughout most of the cosmic history, while the
latter is effective only at z<3 when the IGM is sufficiently enriched by metals
owing to feedback.
