Tracing the Formation of Massive Galaxies in the Early Universe

The average rate at which galaxies are forming stars in the Universe has decreased by more than an order of magnitude over the last 10 billion years. Understanding why certain galaxies shut off their star formation activity, while others do not is one of the key unanswered questions in astrophysics today. Observations in the local Universe suggest that the mechanism responsible for quenching star formation in galaxies may be intimately linked to their structural transformation. In order to test quenching scenarios, however, it is vital to look beyond the local Universe and identify the first generation of quiescent galaxies at high redshift. I will discuss my work studying the first massive systems to appear on the quiescent "red sequence" at redshifts z>1, when the universe was less than half its current age. I will show that the properties of these galaxies are challenging not only our understanding of how star formation is quenched in galaxies, but also how mass is assembled in a hierarchical universe. I will also highlight how future work with the CANDELS survey, as well as the next generation of astronomical facilities coming online over the next 5-10 years, will help establish the primary mechanisms responsible for star-formation quenching and will revolutionize our understanding of how the most massive galaxies in the Universe formed.