When relics were made: vigorous stellar rotation and low dark matter content in the massive ultra-compact galaxy GS-9209 at z = 4.66

James Webb Space Telescope (JWST) uncovered a large number of massive quiescent galaxies(MQGs) at z > 3, which theoretical modelsstruggle to reproduce. Explaining the number density of such objectsrequires extremely high conversion efficiency of baryonsinto starsin early dark matter haloes. Using stellar kinematics, we can investigate the processesshaping the mass assembly histories of MQGs. We present high-resolution JWST/NIRSpec integral field spectroscopy of GS-9209, a massive compact quiescent galaxy at z = 4.66 (log (M∗/M) = 10.52 ± 0.06, Reff = 220 ± 20 pc). Full spectral fitting of the spatially resolved stellar continuum reveals a clear rotational pattern, yielding a spin parameter of λ2Reff = 0.85 ± 0.10. This study suggests that at least a fraction of the earliest quiescent galaxies were fast rotators and that quenching was a dynamically gentle process, preserving the stellar disc even in highly compact objects. Using Jeans anisotropic modelling and assuming a Navarro–Frenk–White profile, we measure a dark matterfraction of fDM (< 2Reff) = 14.5+6.0−4.2 per cent. Our findings use stellar kinematics to confirm the massive nature of early quiescent galaxies, previously inferred from stellar population modelling. We suggest that GS-9209 has a similar structure to low-redshift ‘relic’ galaxies. However, unlike relic galaxies, which have bottom-heavy initial mass functions (IMF), the dynamically inferred stellar mass-to-light ratio of GS-9209 is consistent with a Milky Way-like IMF. The kinematical properties of GS-9209 are different from those of z < 1 early-type galaxies and more similar to those of recently quenched post-starburst galaxies at z > 2.