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The extended stellar halos of galaxies contain important clues forinvestigating their assembly history and evolution. We investigate the resolvedstellar content and the extended halo of NGC 5128 as a function ofgalactocentric distance. We used HST images to resolve individual red giantbranch (RGB) stars in 28 independent pointings. Star counts from deep VIcolor-magnitude diagrams reaching at least 1.5 mag below the tip of the RGB areused to derive the surface density distribution of the halo. The contaminationby Milky Way stars is assessed with a new control field, with models, and bycombining optical and near-IR photometry. We present a new calibration of theWFC3 F606W+F814W photometry to the ground-based VI photometric system. Thephotometry shows that the stellar halo of NGC 5128 is dominated by old RGBstars that are present in all fields. The V-band surface brightness changesfrom 23 to 32 mag/arcsec$^2$ between 8.3 kpc from the galaxy center to ouroutermost halo fields located 140 kpc away from the center along the major axisand 92 kpc along the minor axis. Within ~30 kpc, we also find evidence for a2-3 Gyr old population traced by bright asymptotic giant branch stars. Thispopulation 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 stellarsurface density profile is well fit by a r$^{1/4}$ curve or a power-law $\simr^{-3.1}$ over the full radial range, with no obvious break in the slope, butwith large field-to-field scatter. The ellipticity measured fromintegrated-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 mostdistant field reaches nearly 30 effective radii. [abridged]

The outermost stellar halo of NGC 5128 (Centaurus A): Radial structure