The birth of relics: JWST reveals an ultra-compact fast rotator with little dark matter at z=4.66

Abstract

JWST revealed an over-abundance of z>3 massive quiescent galaxies (MQGs) that theoretical models cannot fully explain. The MQGs number density implies a near-unity conversion efficiency of baryons into stars in early massive halos formed on extremely short timescales. Spatially resolved stellar kinematics can unveil crucial insights into the mass assembly of early MQGs. We present JWST/NIRSpec integral field spectroscopy of GS-9209, an ultra-compact massive (Reff = 220 ± 20 pc; log(M*/Msun)=10.52 ± 0.06) galaxy at z=4.66 with a high rotational support (spin parametre λReff = 0.65 ± 0.12). This suggests a dynamically gentle quenching mechanism. Using Jeans Anisotropic Models, we construct a robust dynamical model of this galaxy and determine that its central region is strongly baryon dominated. With its ultra-compact size, a low dark matter content and fast stellar rotation, GS-9209 is very similar to local relic galaxies, but a direct link between high-redshift MQGs and low-redshift relics remains uncertain.