MgAl burning chain in M 54: The globular cluster-like properties of a nuclear star cluster

In this study, we present the chemical abundances of Fe, Mg, Al, Si, and K for a sample of 233 likely member stars of M 54. All the stars were observed with the FLAMES high-resolution multi-object spectrograph mounted at the VLT. Our analysis confirmed the presence of a large metallicity range in M 54, with the majority of the stars having ̊m -1.8 < Fe/H < -1.0 dex and a few stars having Fe/H $> -1.0$ dex. The mean value of the total sample is ̊m Fe/H = -1.40 (σ = 0.22 dex). A Markov chain Monte Carlo analysis revealed that the observed spread in Fe/H is compatible with a non-null intrinsic iron dispersion. We also found that the metallicity distribution function and the broadening of the red giant branch of M 54 are not compatible with a single age, but instead suggest a wide age range from sim13 Gyr to ̊m $<$0.0 dex) stars are identified in M 54. The evidence collected so far cannot be explained by either a globular cluster-like scenario or galactic chemical evolution. The chemical properties of M 54 can be explained within a scenario in which this system formed through the merging of two globular clusters: a metal-poor one with standard characteristics and a more metal-rich one with more pronounced chemical anomalies, possibly younger than the first one. M 54 is confirmed to be a key stellar system for explaining the chemical evolution of a nuclear star cluster. Gyr or a smaller age range if a significant He enhancement (̊m Y ∼ 0.35/0.40) is present in the most metal-rich stars. We identified among the stars in M 54 an entire pattern of anticorrelations linked to the MgAl burning cycle. In particular, the metal-rich component displays a higher level of H-burning, with the presence of more extended anticorrelations than the metal-poor component. No Mg-poor ( Mg/Fe