重新研究天炉座矮椭球星系中神秘的第六个星团
Revisiting the enigmatic sixth star cluster in the Fornax dwarf spheroidal galaxy
AI总结：
本研究结合MUSE大视场模式观测与深度GMOS-S成像，重新分析天炉座矮椭球星系中的Fornax 6过密区，证实其为正经历潮汐瓦解的低质量球状星团，是该星系球状星团系统中最年轻且金属丰度最高的成员。
AI中文摘要：
天炉座矮椭球星系（dSph）是本星系群中仅有的两个拥有球状星团（GC）族群的矮椭球星系之一，其现今的属性常被用于研究暗物质的本质。最近，一个被称为Fornax 6的额外恒星过密区被认定为天炉座中存在的第六个球状星团，它位于距离星系中心投影距离约0.3千秒差距（kpc）的位置。然而，这一结论是基于浅层、低分辨率的测光观测以及数量有限的光谱得出的。在本研究中，我们通过分析MUSE/大视场模式（Wide-Field-Mode）的观测数据以及深度GMOS-S成像数据，重新探究Fornax 6的本质。利用包含132颗红巨星和水平支恒星的光谱样本，我们证实Fornax 6过密区相对于周围场星族群是一个独特的化学动力学组分。具体而言，我们识别出43颗可能与该星团相关的成员星，据此测得其金属丰度[Fe/H] = -0.61±0.03 dex，视向速度v_los = 50.9^(+0.8)_(-0.7) km s^(-1)，速度弥散σ_v = 3.2^(+1.4)_(-1.5) km s^(-1)。本次研究首次观测到的主序折转点强烈表明，这些恒星是同时代形成的，且能用年龄约为30亿年（Gyr）的等时线很好地描述。该星团的特征包括不规则的形态、较大的半光半径（R_h = 8.8^(+1.1)_(-1.3) pc）、较小的椭率（e = 0.14^(+0.12)_(-0.10)）以及低光度（M_V = -5.0 ± 0.4）。我们对Fornax 6的改进表征支持将其归类为一个可能正处于潮汐瓦解过程中的真正低质量星团，这使其成为天炉座独特球状星团系统中最年轻、金属丰度最高的成员。
英文摘要：
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 $\rm [Fe/H] =-0.61\pm0.03\,$dex, $v_{\rm los} = 50.9^{+0.8}_{-0.7}\,\rm km \,s^{-1}$, $σ_v = 3.2^{+1.4}_{-1.5}\,\rm km \,s^{-1}$. The main-sequence turn-off, observed here for the first time, strongly suggests that these stars are coeval and well described by a $\sim$3 Gyr old isochrone. The cluster is characterised by an irregular morphology, a large half-light radius ($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 \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.
