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

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 ∼5-14R_eff). 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 ∼14R_eff), 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.