The outermost stellar halo of NGC 5128 (Centaurus A): Radial structure
  The extended stellar halos of galaxies contain important clues for
investigating their assembly history and evolution. We investigate the resolved
stellar content and the extended halo of NGC 5128 as a function of
galactocentric distance. We used HST images to resolve individual red giant
branch (RGB) stars in 28 independent pointings. Star counts from deep VI
color-magnitude diagrams reaching at least 1.5 mag below the tip of the RGB are
used to derive the surface density distribution of the halo. The contamination
by Milky Way stars is assessed with a new control field, with models, and by
combining optical and near-IR photometry. We present a new calibration of the
WFC3 F606W+F814W photometry to the ground-based VI photometric system. The
photometry shows that the stellar halo of NGC 5128 is dominated by old RGB
stars that are present in all fields. The V-band surface brightness changes
from 23 to 32 mag/arcsec$^2$ between 8.3 kpc from the galaxy center to our
outermost halo fields located 140 kpc away from the center along the major axis
and 92 kpc along the minor axis. Within ~30 kpc, we also find evidence for a
2-3 Gyr old population traced by bright asymptotic giant branch stars. This
population contributes only up to 10% in total stellar mass if it is 2 Gyr old,
but a larger fraction of 30-40% is required if its age is 3 Gyr. The stellar
surface density profile is well fit by a r$^{1/4}$ curve or a power-law $\sim
r^{-3.1}$ over the full radial range, with no obvious break in the slope, but
with large field-to-field scatter. The ellipticity measured from
integrated-light photometry in the inner parts, $e=(b/a)=0.77$, flattens to
$e=0.54 \pm 0.02$ beyond 30 kpc. Considering the flattening of the outer halo,
the projection of the elliptical isophote on the semimajor axis for our most
distant field reaches nearly 30 effective radii. [abridged]
