The outer halo of the nearest giant elliptical: a VLT/VIMOS survey of the resolved stellar populations in Centaurus A to 85 kpc★

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Abstract
We present the first survey of resolved stellar populations in the remote outer halo of our nearest giant elliptical (gE), Centaurus A (D = 3.8 Mpc). Using the VIsible Multi Object Spectrograph (VIMOS)/Very Large Telescope (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 ∼5–14Reff). We use resolved star counts to map the spatial and colour distribution of red giant branch (RGB) stars down to ∼2 mag below the RGB tip. We detect an extended halo out to the furthermost elliptical radius probed (∼85 kpc or ∼14Reff), demonstrating the vast extent of this system. We detect a localized 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 ≈ −0.002-0.004 dex kpc−1, and the fraction of metal-poor stars (defined as [Fe/H] < −1.0) is ≈40–50 per cent at all radii. We discuss these findings in the context of galaxy formation models for the buildup of gE haloes.
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