Neutron star mergers are one of the most promising astrophysical sites of r-process
elements such as gold, platinum, and europium. Recent astronomical observations have
shown that there are large star-to-star scatters in the abundances of r-process elements
in extremely metal-poor stars. However, the enrichment of r-process elements by neutron
star mergers is not yet clear. In this talk, we report the results of high-resolution
N-body/smoothed particle hydrodynamics simulations of the Local Group galaxies. We find
that galaxies with star formation rates less than 0.001 Msun per year reproduce the
observed abundances of r-process elements. On the other hand, r-process elements appear
at a higher metallicity in galaxies with star formation rates greater than 0.01 Msun per
year. The scatters of heavy elements mainly come from the inhomogeneity of the metals in
the interstellar medium. We find that timescale of metal mixing is less than 40
Myr. This timescale is lower than that of typical dynamical times of dwarf galaxies. We
also show that the r-process enhanced stars seen in the Milky Way halo are formed in
halos with a size of ultra-faint dwarf galaxies. Our results demonstrate that the future
observations of r-process elements in extremely metal-poor stars will be able to
constrain the early chemo-dynamical evolution of the Local Group galaxies.