Are small-scale sub-structures a universal property of galaxy halos? The case of the giant elliptical NGC~5128

TL;DR

This study investigates the presence of small-scale sub-structures in the stellar halo of galaxy NGC 5128, finding no spatial sub-structures but significant localized metallicity variations, suggesting such features may be common in large galaxy halos.

Contribution

It provides the first detailed analysis of both spatial and chemical sub-structures in NGC 5128's halo, highlighting the prevalence of chemical inhomogeneities over spatial ones.

Findings

No statistically significant spatial sub-structures detected above 100 pc scale.

Significant localized metallicity inhomogeneities observed.

Chemical substructures may originate from incomplete mixing of disrupted low-mass systems.

Abstract

We present an analysis of the spatial and chemical sub-structures in a remote halo field in the nearby giant elliptical galaxy Centaurus A (NGC~5128), situated at about 38 kpc from the centre of the galaxy. The observations were taken with the Advanced Camera for Surveys instrument on board the Hubble Space Telescope, and reach down to the horizontal branch. In this relatively small 3.8 kpc by 3.8 kpc field, after correcting for Poisson noise, we do not find any statistically strong evidence for the presence of small-scale sub-structures in the stellar spatial distribution on scales greater than 100 pc. However, we do detect the presence of significant small spatial-scale inhomogeneities in the stellar median metallicity over the surveyed field. We argue that these localized chemical substructures could be associated with not-fully mixed debris from the disruption of low mass systems. NGC 5128 joins the ranks of the late-type spiral galaxies the Milky Way, for which the stellar halo appears to be dominated by small-scale spatial sub-structures, and NGC~891, where localized metallicity variations have been detected in the inner extra-planar regions. This suggests that the presence of small-scale sub-structures may be a generic property of stellar halos of large galaxies.