Ripe Fruits on the Cosmic Vine: What Massive Quiescent Galaxies at z>3 Reveal about Cluster Formation

The formation of the brightest cluster galaxies (BCGs) and their progenitors (proto-BCGs) is crucial to our understanding of cluster formation. Therefore, it is important to determine when and how the first generation of massive quiescent galaxies formed in proto-clusters. In this talk, I will report the discovery of the large-scale structure "Cosmic Vine" at z=3.44, and elaborate how it impacts on our understanding of BCG formation. Briefly, we found the two most massive galaxies of the Cosmic Vine to be quiescent with bulge-dominated morphologies. Comparisons with simulations suggest that the Cosmic Vine would form a massive cluster, and the two massive galaxies are likely forming the BCGs. The results unambiguously reveal that massive quiescent galaxies can form in growing large-scale structures at z>3, disfavoring the environmental quenching mechanisms that require a virialized cluster core. Instead, these massive quiescent galaxies are likely quenched by merger-triggered starburst or AGN feedback before falling into a cluster core. Moreover, the observed specific star formation rates of massive quiescent galaxies in z>3 dense environments are one to two orders of magnitude lower than what simulation predicted, posing a potential challenge to the models of massive cluster galaxy formation.