Internal kinematics of stellar populations in M54 and in the Sagittarius dwarf galaxy.

Recent work discovered two distinct classes of Type I and Type II Globular Clusters (GCs). While Type I GCs are monometallic, the metallicity spread is a distinctive feature of Type II GCs, which indicates that these clusters have been much more massive at formation and retained the fast ejecta of SNe. It has been suggested that Type II GCs are the remnants of much-more massive stellar systems, like dwarf galaxies that have been cannibalized by the interaction with the Milky Way. Hence, they may play an important role in the assembly of the Galactic Halo. Furthermore, they may increase the number of satellites orbiting the Galaxy and mitigate the missing satellite problem. M 54 is considered the ’Rosetta Stone’ to connect GCs and dwarf galaxies. Indeed, this massive Type II GC is located in the nucleus of the Sagittarius dwarf galaxy and provides the unique opportunity to constrain multiple populations in the galaxy and in the cluster. The internal kinematics of stars in GCs may retain fossil information about the formation process of multiple populations and the connection with the host dwarf galaxy. However, while multiple populations in Type II GCs have been widely investigated by means of photometry and spectroscopy their internal motions are nearly unexplored. Indeed, it is challenging to derive accurate proper motions of stars within distant GCs. In this work, I analyze multi-epoch images of M 54 collected by the Hubble Space Telescope through five filters. The exquisite multi-band photometry that I derived, allowed me to disentangle the main stellar populations in the Sagittarius galaxy and within M 54. Specifically, I identified a first generation 1G, composed of metal-poor stars with pristine helium abundance, a second generation composed of metal-poor stars enhanced in N and He, and a Fe-rich population made of N-rich He-rich stars. By comparing the position of stars at different epochs, I derive high-precision proper motions, that allowed me to measure, for the first time, the internal motions along the plane of the sky for stellar populations in the galaxy and in M 54. I find that all stellar populations in the GC and the galaxy share the same mean motion. Moreover, I determined and investigated the velocity dispersion profiles of the stellar populations within M 54 and within the galaxy. It results that stellar populations in M 54 and in the galaxy exhibit different velocity profiles, thus indicating that they had different origin. I provide the first analysis of the motions of the three main populations within M 54. On the radial axis the 1G, the 2G and the Fedrich population show similar trends, while on the tangential axis the 1G deviates from the behaviour of both the 2G and the Fe-rich population. This finding suggests that stellar populations with different nitrogen abundances share similar motions regardless their metallicity. I discuss these results in the context of the formation scenarios of multiple populations.