Tracing the outer halo in a giant elliptical to 25 Reff
  We have used the ACS and WFC3 cameras on board HST to resolve stars in the
halo of the nearest giant elliptical (gE) galaxy NGC 5128 out to a projected
distance of 140 kpc (25 effective radii, Reff) along the major axis and 90 kpc
(16 Reff) along the minor axis. This dataset provides an unprecedented radial
coverage of the stellar halo properties in any gE galaxy. Color-magnitude
diagrams clearly reveal the presence of the red giant branch stars belonging to
the halo of NGC 5128, even in our most distant fields. The star counts
demonstrate increasing flattening of the outer halo, which is elongated along
the major axis of the galaxy. The V-I colors of the red giants enable us to
measure the metallicity distribution in each field and so map the gradient out
to ~16 Reff from the galaxy center along the major axis. A median metallicity
is obtained even for the outermost fields along both axes. We observe a smooth
transition from a metal-rich ([M/H]~0.0) inner galaxy to lower metallicity in
the outer halo, with the metallicity gradient slope along the major axis of
$\Delta$[M/H]/$\Delta$ R=-0.0054 $\pm$ 0.0006 dex/kpc. In the outer halo,
beyond ~10 Reff, the number density profile follows a power law, but also
significant field-to-field metallicity and star count variations are detected.
The metal-rich component dominates in all observed fields, and the median
metallicity is [M/H]>-1 dex in all fields.
