Witnessing the birth of the most massive galaxies in the Universe

The rich diversity of galaxy morphologies grows out of complex physical processes that govern the formation of new stars and the assembly of stellar mass over time. The advent of new near-infrared facilities allowed us to extensively study the distribution of stellar types and chemical properties of distant massive galaxies by measuring the absorption features in their spectra. This constrained their formation times and provided a more detailed picture of their stellar mass distribution, and their dynamical state at the time when the Universe was less than 4 billion years old.