A faint extended cluster in the outskirts of NGC 5128: evidence of a low mass accretion

TL;DR

This paper reports the discovery of a large, faint, extended globular cluster in the outskirts of NGC 5128, providing evidence for low-mass accretion events and expanding understanding of halo stellar populations.

Contribution

It presents the first confirmed detection of a faint, diffuse globular cluster in the outer halo of a giant galaxy, with detailed analysis of its properties and implications for galaxy accretion history.

Findings

The cluster has a half-light radius of ~17pc and an absolute magnitude of -5.2.

The stellar population is older than 8 Gyr with intermediate metallicity.

A second, fainter cluster candidate was also identified.

Abstract

We report the discovery of an extended globular cluster in a halo field in Centaurus A (NGC 5128), situated $\sim 38\kpc$ from the centre of that galaxy, imaged with the Advanced Camera for Surveys on board the Hubble Space Telescope. At the distance of the galaxy, the half-light radius of the cluster is r_h ~ 17pc, placing it among the largest globular clusters known. The faint absolute magnitude of the star cluster, M_(V,o)=-5.2, and its large size render this object somewhat different from the population of extended globular clusters previously reported, making it the first firm detection in the outskirts of a giant galaxy of an analogue of the faint, diffuse globular clusters present in the outer halo of the Milky Way. The colour-magnitude diagram of the cluster, covering approximately the brightest four magnitudes of the red giant branch, is consistent with an ancient, i.e., older than ~8 Gyr, intermediate-metallicity, i.e., [M/H] ~-1.0 dex, stellar population. We also report the detection of a second, even fainter cluster candidate which would have r_h ~ 9pc, and M_(V,o)=-3.4 if it is at the distance of NGC 5128. The properties of the extended globular cluster and the diffuse stellar populations in its close vicinity suggest that they are part of a low mass accretion in the outer regions of NGC 5128.