2022ApJ...936..136Z

We present the stellar radial velocity analysis of the central $1^{\prime} \times 1^{\prime} $ of the young massive Small Magellanic Cloud star cluster NGC 346. Using VLT/MUSE integral field spectroscopy in combination with Hubble Space Telescope photometry, we extract 103 spectra of cluster member stars suited to measure accurate line-of-sight kinematics. The cluster member stars show two distinct velocity groups at ${v}_{1}=-{3.3}_{-0.2}^{+0.3}\,\mathrm{km}\,{{\rm{s}}}^{-1}$ and ${v}_{2}={2.6}_{-0.1}^{+0.1}\,\mathrm{km}\,{{\rm{s}}}^{-1}$, relative to the systemic velocity of (165.5 ± 0.2) km s^–1, and hint at a third group at ${v}_{3}={9.4}_{-0.1}^{+0.1}\,\mathrm{km}\,{{\rm{s}}}^{-1}$. We show that there is neither a correlation between the velocity groups and the spatial location of the stars, nor their locus on optical color-magnitude diagrams, which makes the stellar velocity a key parameter to separate individual stellar components in such a young star cluster. Velocity group 2 shows clear rotation with Ω₂ =(-0.4 ± 0.1) Myr^–1, corresponding to (-4.9 ± 0.7) km s^–1 at radial distance of 10 pc from the center, a possible remnant of the formation process of NGC 346 through the hierarchical collapse of the giant molecular cloud. The ionizing gas has lost any natal kinematic imprint and shows clear expansion, driven by far-ultraviolet fluxes and stellar winds of the numerous OB stars in the cluster center. The size of this expanding bubble and its expansion velocity of 7.9 km s^–1 are in excellent agreement with the estimate that the latest star formation episode occurred about two million years ago.