Efficient Formation of a Massive Quiescent Galaxy at Redshift 4.9

Within the established framework of structure formation, galaxies start assystems of low stellar mass and gradually grow into far more massive galaxies.The existence of massive galaxies in the first billion years of the Universe,suggested by recent observations, appears to challenge this model, as suchgalaxies would require highly efficient conversion of baryons into stars. Aneven greater challenge in this epoch is the existence of massive galaxies thathave already ceased forming stars. However, robust detections of early massivequiescent galaxies have been challenging due to the coarse wavelength samplingof photometric surveys. Here we report the spectroscopic confirmation with theJames Webb Space Telescope of the quiescent galaxy RUBIES-EGS-QG-1 at redshiftz=4.896, 1.2 billion years after the Big Bang. Deep stellar absorptionfeatures in the spectrum reveal that the galaxy's stellar mass of10^10.9 M_⊙, corroborated by the mass implied by its gas kinematics,formed in a short 340Myr burst of star formation, after which starformation activity dropped rapidly and persistently. According to currentgalaxy formation models, systems with such rapid stellar mass growth and earlyquenching are too rare to plausibly occur in the small area probedspectroscopically with JWST. Instead, the discovery of RUBIES-EGS-QG-1 impliesthat early massive quiescent galaxies can be quenched earlier or exhaust gasavailable for star formation more efficiently than currently assumed.