A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. II. Kinematic Characterization of the Stellar Populations

Abstract

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 ∼ 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 (∼ 0.8 km s −1 ) and with a velocity dispersion that follows a Plummer profile. The YMR population displays a high amount of rotation (∼ 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 (∼ 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).

The accreted nuclear clusters of the Milky Way

“…On the other hand, the young metal-rich populations appears to be the most centrally concentrated, and hence clearly associated with M54. This is also supported by the kinematics of the populations (see Alfaro-Cuello et al 2020).…”

Testing for Dark Matter in the Outskirts of Globular Clusters

“…The velocity dispersion of M54 begins to rise at about 5 half-mass radii (Bellazzini et al 2008;Ibata et al 2009) likely due to the dwarf galaxyʼs remnant dark matter halo. M54 is not typical, since it qualifies as a nuclear star cluster, having both an extended old stellar population and younger stars in the inner region (Alfaro-Cuello et al 2020). More generally, the lower-mass nuclear star clusters are considered to be a globular cluster (or the merger of several) in the centers of dark halos (Neumayer et al 2020).…”

On the Origin of a Rotating Metal-poor Stellar Population in the Milky Way Nuclear Cluster

“…We note that our SC model can also be interpreted as the remaining nucleus of a dwarf galaxy that merged with the MW and lost its stellar envelope, a scenario suggested to explain the properties of several globular clusters observed in the MW halo, including ωCen (Hilker & Richtler 2000) and M54 (Alfaro-Cuello et al 2019. We stress that the numerical setup adopted here does not rely on any assumption on the earlier SC evolution.…”