A Massive Quiescent Galaxy in a Group Environment at z=4.53

We report on the spectroscopic confirmation of a massive quiescent galaxy at z(spec) = 4.53 in the COSMOS field. The object was first identified as a galaxy with suppressed star formation at z(phot) similar to 4.65 from the COSMOS2020 catalog. The follow-up spectroscopy with Keck/MOSFIRE in the K band reveals faint [O II] emission and the Balmer break, indicative of evolved stellar populations. We fit the spectral energy distribution using photometry and a spectrum to infer physical properties. The obtained stellar mass is high (M-* similar to 10(10.8) M-circle dot) and the current star formation rate is more than 1 dex below that of main-sequence galaxies at z = 4.5. Its star formation history suggests that this galaxy experienced rapid quenching from z similar to 5. The galaxy is among the youngest quiescent galaxies confirmed so far at z(spec) > 3 with z(form) similar to 5.2 (200 Myr ago), which is the epoch when 50% of the total stellar mass was formed. A unique aspect of the galaxy is that it is in an extremely dense region; there are four massive star-forming galaxies at 4.4 < z(phot) < 4.7 located within 150 physical kpc from the galaxy. Interestingly, three of them have virial radii that strongly overlap with that of the central quiescent galaxy (similar to 70 kpc), suggesting that the overdensity region is likely the highest-redshift candidate of a dense group with a spectroscopically confirmed quiescent galaxy at the center. The group provides us with a unique opportunity to gain insights into the role of the group environment in quenching at z similar to 5, which corresponds to the formation epoch of massive elliptical galaxies in the local Universe.