The Outer Halo of the Nearest Giant Elliptical: A VLT/VIMOS Survey of the Resolved Stellar Populations in Centaurus A to 85 kpc

TL;DR

This study uses deep VLT/VIMOS imaging to map the extended stellar halo of Centaurus A out to 85 kpc, revealing its vast size, metallicity distribution, and substructures, providing insights into galaxy formation.

Contribution

First detailed resolved stellar population survey of Centaurus A's outer halo, extending to 85 kpc, with analysis of its structure, metallicity, and substructures.

Findings

Extended halo detected out to 85 kpc (~14 R_{eff}).

Halo shows high median metallicity with little radial variation.

Substructure and changes in halo shape observed.

Abstract

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.