Early-type galaxies in low-density environments: NGC 6876 explored through its globular cluster system

Abstract

We present the results of a photometric study of the early-type galaxy NGC 6876 and the surrounding globular cluster system (GCS). The host galaxy is a massive elliptical, the brightest of this type in the Pavo Group. According to its intrinsic brightness (Mv~-22.7), it is expected to belong to a galaxy cluster instead of a poor group. Observational material consists of g', r', i' images obtained with the Gemini/GMOS camera. The selected globular cluster (GC) candidates present a clear bimodal colour distribution at different galactocentric radii, with mean colours and dispersions for the metal-poor ("blue") and metal-rich ("red") typical of old GCs. The red subpopulation dominates close to the galaxy centre, in addition to the radial projected distribution showing that they are more concentrated towards the galaxy centre. The azimuthal projected distribution shows an overdensity in the red subpopulation in the direction of a trail observed in X-ray that could be evidence of interactions with its spiral neighbour NGC 6872. The turn-over of the luminosity function gives an estimated distance modulus (m - M) ≈ 33.5 and the total population amounts to 9400 GCs, i.e. a quite populous system. The halo mass obtained using the number ratio (i.e. the number of GCs with respect to the baryonic and dark mass) gives a total of ~10^13, meaning it is a very massive galaxy, given the environment.