Revisiting the enigmatic sixth star cluster in the Fornax dwarf spheroidal galaxy

The Fornax dwarf spheroidal (dSph) galaxy is one of only two Local Group dSphs that host a population of globular star clusters (GCs), the present-day properties of which have often been used to investigate the nature of the dark matter. An additional overdensity of stars called Fornax 6, lying at a projected distance of about 0.3 kpc from the galaxy centre, was recently identified as a sixth GC residing in Fornax. However, this conclusion was based on shallow, low-resolution photometric observations and a limited number of spectra. Here, we reinvestigate the nature of Fornax 6 by analysing MUSE/Wide-Field-Mode observations alongside deep GMOS-S imagery. Using a sample of 132 spectra of red giant and horizontal branch stars, we confirm the Fornax 6 overdensity as a distinct chemo-dynamical component with respect to the surrounding field population. Specifically, we identify 43 likely members associated with the cluster, from which we measure [Fe/H] =-0.61 plus or minus0.03, dex, v los = 50.9+0.8-0.7, km,s-1, σ v = 3.2+1.4-1.5, km,s-1. The main-sequence turn-off, observed here for the first time, strongly suggests that these stars are coeval and well described by a about 3 Gyr old isochrone. The cluster is characterised by an irregular morphology, a large half-light radius (R h=8.8+1.1-1.3, pc), a small flattening (e=0.14+0.12-0.10), and low luminosity (M V = -5.0 plus or minus 0.4). Our improved characterisation of Fornax 6 supports its classification as a genuine low-mass cluster likely undergoing tidal disruption, making it the youngest and most metal-rich member of Fornax's unique GC system.