The Sagittarius dwarf spheroidal galaxy ( Sgr dSph ) is in an advanced stage of disruption but still hosts its nuclear star cluster ( NSC ) , M54 , at its center .
In this paper , we present a detailed kinematic characterization of the three stellar populations present in M54 : young metal-rich ( YMR ) ; intermediate-age metal-rich ( IMR ) ; and old metal-poor ( OMP ) , based on the spectra of \sim 6500 individual M54 member stars extracted from a large MUSE/VLT dataset .
We find that the OMP population is slightly flattened with a low amount of rotation ( \sim 0.8 km s ^ { -1 } ) and with a velocity dispersion that follows a Plummer profile .
The YMR population displays a high amount of rotation ( \sim 5 km s ^ { -1 } ) and a high degree of flattening , with a lower and flat velocity dispersion profile .
The IMR population shows a high but flat velocity dispersion profile , with some degree of rotation ( \sim 2 km s ^ { -1 } ) .
We complement our MUSE data with information from Gaia DR2 and confirm that the stars from the OMP and YMR populations are comoving in 3D space , suggesting that they are dynamically bound .
While dynamical evolutionary effects ( e.g .
energy equipartition ) are able to explain the differences in velocity dispersion between the stellar populations , the strong differences in rotation indicate different formation paths for the populations , as supported by an N -body simulation tailored to emulate the YMR-OMP system .
This study provides additional evidence for the M54 formation scenario proposed in our previous work , where this NSC formed via GC accretion ( OMP ) and in situ formation from gas accretion in a rotationally supported disc ( YMR ) .