2025A&A...699A..94B

Context. Ultra-diffuse galaxies (UDGs) are an intriguing population of galaxies. Despite their dwarf-like stellar masses and low surface brightness, they have large half-light radii and exhibit a diverse range of globular cluster (GC) populations. Some UDGs host many GCs while others have none, raising questions about the conditions under which star clusters form in dwarf galaxies. GAMA 526784, an isolated UDG with both an old stellar body and an extended star-forming front, including many young star clusters, provides an exceptional case to explore the link between UDG evolution and star cluster formation.
Aims. This study investigates the stellar populations, star clusters, ionised gas properties, and kinematics of GAMA 526784, focusing on the galaxy's potential to form massive GCs and its connection to broader UDG formation scenarios.
Methods. Imaging from HST and Subaru/HSC, alongside MUSE spectroscopy, were used to analyse the galaxy's morphology, chemical composition, and kinematics. A combination of SED fitting and full spectral fitting was applied.
Results. GAMA 526784's central stellar body exhibits a low-metallicity ([M/H] ∼-1.0 dex) and an old age (t_M ∼9.9 Gyr), indicative of a quiescent core. The outskirts are much younger (t_M ∼0.9 Gyr), but slightly more metal-poor ([M/H] ∼-1.2 dex). The stellar kinematics show low velocity dispersions (∼10 km s^–1) and a coherent rotational field, while the ionised gas exhibits higher dispersions (reaching ∼50 km s^–1), a misaligned rotation axis (∼20^∘) and localised star formation, what could be suggestive of a recent interaction. The young star clusters span ages of 8-11 Myr and masses of log(M_⋆/M_⊙) ∼5.0, while the old GC candidates have ∼9 Gyr and stellar masses of log(M_⋆/M_⊙) ∼5.5.
Conclusions. GAMA 526784's properties point to interactions that triggered localised star formation, leading to the formation of young star clusters. Future observations of its molecular and neutral gas content will help assess its environment, the trigger of this star-forming episode, and explore its potential to sustain star formation.