An Ancient Massive Quiescent Galaxy Found in a Gas-rich z ∼ 3 Group

Abstract 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 10 11 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 Ly α halo, the existence of this galaxy demonstrates that gas accretion shutdown is not necessary for quenching and its maintenance.