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]