Are small-scale sub-structures a universal property of galaxy halos? The
  case of the giant elliptical NGC~5128
  We present an analysis of the spatial and chemical sub-structures in a remote
halo field in the nearby giant elliptical galaxy Centaurus A (NGC~5128),
situated at about 38 kpc from the centre of the galaxy. The observations were
taken with the Advanced Camera for Surveys instrument on board the Hubble Space
Telescope, and reach down to the horizontal branch. In this relatively small
3.8 kpc by 3.8 kpc field, after correcting for Poisson noise, we do not find
any statistically strong evidence for the presence of small-scale
sub-structures in the stellar spatial distribution on scales greater than 100
pc. However, we do detect the presence of significant small spatial-scale
inhomogeneities in the stellar median metallicity over the surveyed field. We
argue that these localized chemical substructures could be associated with
not-fully mixed debris from the disruption of low mass systems. NGC 5128 joins
the ranks of the late-type spiral galaxies the Milky Way, for which the stellar
halo appears to be dominated by small-scale spatial sub-structures, and
NGC~891, where localized metallicity variations have been detected in the inner
extra-planar regions. This suggests that the presence of small-scale
sub-structures may be a generic property of stellar halos of large galaxies.
