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

Abstract

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 $\sim$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 $[Fe/H]=-0.61\pm 0.03$dex, $v_{\mathrm{los}}=50.9_{-0.7}^{+0.8}\mathrm{km}{\mathrm{s}}^{-1}$, ${\sigma }_v=3.2_{-1.5}^{+1.4}\mathrm{km}{\mathrm{s}}^{-1}$. The main-sequence turn-off, observed here for the first time, strongly suggests that these stars are coeval and well described by a $\sim$3 Gyr old isochrone. The cluster is characterised by an irregular morphology, a large half-light radius ($R_h=8.8_{-1.3}^{+1.1}$pc), a small flattening ($e=0.14_{-0.10}^{+0.12}$), and low luminosity ($M_V=-5.0\pm 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.

Submitted to MNRAS. 18 pages, 13 figures, 2 tables