The most distant galaxies detected were seen when the Universe was a scant 5% of its current age. At these times, progenitors of galaxies such as the Milky Way were about 10,000 times less massive. Using the James Webb Space Telescope (JWST) combined with magnification from gravitational lensing, these low-mass galaxies can not only be detected but also be studied in detail. Here we present JWST observations of a strongly lensed galaxy at z_spec = 8.296 ± 0.001, showing massive star clusters (the Firefly Sparkle) cocooned in a diffuse arc in the Canadian Unbiased Cluster Survey (CANUCS)¹. The Firefly Sparkle exhibits traits of a young, gas-rich galaxy in its early formation stage. The mass of the galaxy is concentrated in 10 star clusters (49–57% of total mass), with individual masses ranging from 10⁵M_⊙ to 10⁶M_⊙. These unresolved clusters have high surface densities (>10³M_⊙ pc⁻²), exceeding those of Milky Way globular clusters and young star clusters in nearby galaxies. The central cluster shows a nebular-dominated spectrum, low metallicity, high gas density and high electron temperature, hinting at a top-heavy initial mass function. These observations provide our first spectrophotometric view of a typical galaxy in its early stages, in a 600-million-year-old Universe.

探测到的最遥远的星系是在宇宙年龄仅为当前年龄的 5% 时看到的。在这些时候，银河系等星系的祖先的质量大约小了 10,000 倍。使用詹姆斯韦伯太空望远镜 （JWST） 结合引力透镜的放大率，不仅可以探测到这些低质量星系，还可以对其进行详细研究。在这里，我们展示了 z_spec = 8.296 ± 0.001 的强透镜星系的 JWST 观测结果，显示了在加拿大无偏星团巡天 （CANUCS^{） 1} 中被包裹在弥散弧中的大质量星团（萤火虫火花）。Firefly Sparkle 在其早期形成阶段表现出年轻、富含气体的星系的特征。星系的质量集中在 10 个星团中（占总质量的 49-57%），单个质量从 10⁵M_⊙ 到 10⁶M_⊙ 不等。这些未分辨的星团具有很高的表面密度（>10³M_⊙ pc⁻²），超过了附近星系中的银河系球状星团和年轻星团。中心星团显示出以星球为主的光谱、低金属丰度、高气体密度和高电子温度，暗示了头重脚轻的初始质量函数。这些观测结果为我们提供了第一个分光光度法视图，即 6 亿年前的宇宙中一个处于早期阶段的典型星系。