2021ApJ...917L..17K

Deep Atacama Large Millimeter/submillimeter Array and Hubble Space Telescope observations reveal the presence of a quenched massive galaxy within the z = 2.91 galaxy group RO-1001. With a mass-weighted stellar age of 1.6 ± 0.4 Gyr this galaxy is one of the oldest known at z ∼ 3, implying that most of its 10¹¹ M_☉ of stars were rapidly formed at z > 6-8. This is a unique example of the predominantly passive evolution of a galaxy over at least 3 < z < 6 following its high-redshift quenching and a smoking-gun event pointing to the early imprint of an age-environment relation. At the same time, being in a dense group environment with extensive cold gas reservoirs as betrayed by a giant Lyα halo, the existence of this galaxy demonstrates that gas accretion shutdown is not necessary for quenching and its maintenance.