  Stellar metallicity distribution functions (MDF) have been measured for
resolved stellar populations in the outer halos of many galaxies in nearby
groups. Among them, the MDF of NGC 5128, the central giant elliptical in the
Centaurus group, provides essential constraints for theories of massive galaxy
formation and hierarchical assembly. To investigate the formation and chemical
evolution history of the outer halo of giant elliptical galaxies, we examine
the chemical properties of three zoom-in high resolution cosmological
hydrodynamical simulations of an NGC 5128-like giant elliptical galaxy and
compare their outer halo MDFs to the observed one of NGC 5128. Even though the
simulated galaxies have different merging histories and age distributions, all
predicted MDFs are in good qualitative agreement with the observed one. The
median metallicity of the simulated galaxies is on average $\rm [M/H]=-0.41 \pm
0.06$ compared to the observed value of $\rm [M/H]=-0.38 \pm 0.02$ for NGC
5128, and the dispersion in metallicity is $\sim 0.77$ dex for both observed
and simulated galaxies. We investigate the origin of the stars ending up in the
outer halo field of simulated galaxies and show that most have an `accreted'
origin, formed in other small galaxies and later accreted in mergers. Only
$\sim 15$ percent of the stars are formed `in situ' within the main progenitor
of galaxy and radially migrate outwards. We show that the contribution of
metal-rich in situ stars is sub-dominant in the outer halos of our simulated
galaxies, but can be prominent in the inner regions.
