James Webb Space Telescope Feed Post

Efficient formation of a massive quiescent galaxy at redshift 4.9

Spatial clustering of spectroscopically-confirmed sources at z ˜ 4.90 (circles) around RUBIES-EGS-QG-1 (red star). The submillimeter galaxy, the brightest source among the group of 4 at a projected distance of 16 comoving Mpc, is indicated by a purple triangle. The background image shows the NIRCam F444W mosaic of the EGS field. Compared to the typical projected spatial clustering (within redshift ranges ?z = 0.03) and redshift clustering (within apertures of radius < 3 Mpc) for galaxies in the redshift range 4 < z < 6 with robust redshifts from JWST spectroscopy, RUBIES-EGS-QG-1 clearly resides in an overdense environment, forming the highest redshift known overdensity hosting a massive quiescent galaxy. We show false-color images (created from NIRCam F150W, F277W and F444W images) of RUBIES-EGS-QG-1 and its 6 nearby neighbors. Abstract: Within the established framework of structure formation, galaxies start as systems 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 such galaxies would require highly efficient conversion of baryons into stars. An even greater challenge in this epoch is the existence of massive galaxies that have already ceased forming stars. However, robust detections of early massive quiescent galaxies have been challenging due to the coarse wavelength sampling of photometric surveys. Here we report the spectroscopic confirmation with the James Webb Space Telescope of the quiescent galaxy RUBIES-EGS-QG-1 at redshift z=4.896, 1.2 billion years after the Big Bang. Deep stellar absorption features in the spectrum reveal that the galaxy's stellar mass of 1010.9M?, corroborated by the mass implied by its gas kinematics, formed in a short 340Myr burst of star formation, after which star formation activity dropped rapidly and persistently. According to current galaxy formation models, systems with such rapid stellar mass growth and early quenching are too rare to plausibly occur in the small area probed spectroscopically with JWST. Instead, the discovery of RUBIES-EGS-QG-1 implies that early massive quiescent galaxies can be quenched earlier or exhaust gas available for star formation more efficiently than currently assumed.