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Multiple populations in the Sagittarius nuclear cluster M54 and in other anomalous globular clusters

M54 is the central cluster of the Sagittarius dwarf galaxy. This stellar system is now in process of being disrupted by the tidal interaction with the Milky Way and represents one of the building blocks of the Galactic Halo. Recent discoveries, based on the synergy of photometry and spectroscopy have revealed that the color-magnitude diagram of some massive, anomalous, Globular Clusters (GCs) host stellar populations with different content of heavy elements.In this paper, I use multi-wavelength Hubble Space Telescope photometry to detect and characterize multiple stellar populations in M54. I provide empirical evidence that this GC shares photometric and spectroscopic similarities with the class of anomalous GCs. These findings make it tempting to speculate that, similarly to Sagittarius nuclear cluster M54, other anomalous GCs were born in an extra-Galactic environment.