The Outer Halo of the Nearest Giant Elliptical: A VLT/VIMOS Survey of
  the Resolved Stellar Populations in Centaurus A to 85 kpc
  We present the first deep survey of resolved stellar populations in the
remote outer halo of our nearest giant elliptical (gE), Centaurus A (D=3.8
Mpc). Using the VIMOS/VLT optical camera, we obtained deep photometry for four
fields along the major and minor axes at projected elliptical radii of ~30-85
kpc (corresponding to ~5-14 R_{eff}). We use resolved star counts to map the
spatial and colour distribution of red giant branch (RGB) stars down to ~2
magnitudes below the RGB tip. We detect an extended halo out to the furthermost
elliptical radius probed (~85 kpc or ~14 R_{eff}), demonstrating the vast
extent of this system. We detect a localised substructure in these parts,
visible in both (old) RGB and (intermediate-age) luminous asymptotic giant
branch stars, and there is some evidence that the outer halo becomes more
elliptical and has a shallower surface brightness profile. We derive
photometric metallicity distribution functions for halo RGB stars and find
relatively high median metallicity values ([Fe/H]_{med} -0.9 to -1.0 dex) that
change very little with radius over the extent of our survey. Radial
metallicity gradients are measured to be ~-0.002 to -0.004 dex/kpc and the
fraction of metal-poor stars (defined as [Fe/H]<-1.0) is ~40-50% at all radii.
We discuss these findings in the context of galaxy formation models for the
buildup of gE haloes.
