FIRST-GIANTS · Early Giants in Context: How could Galaxies Grow so Rapidly in the First Billion Years?

The James Webb Space Telescope (JWST) has revealed that galaxies in the first billion years of the Universe are maturing surprisingly fast. Massive galaxies outnumber predictions, appear to have high stellar mass densities, host massive black holes and in some cases disky morphologies.Understanding this rapid assembly is crucial: the abundance of early massive galaxies would pose a major challenge for current galaxy formation models in the ΛCDM framework. However, these results remain contentious, as they are primarily based on broad-band photometric samples, interpreted with stellar population models that were calibrated in the local Universe. The masses, nature and space density of the candidate massive galaxies therefore are highly uncertain.To determine how galaxies could grow so rapidly requires a comprehensive approach: high-quality spectra for a large set of candidate massive galaxies, combined with novel modelling to obtain dynamical masses, robust stellar masses and star formation histories, and to identify active galactic nuclei (AGN). As PI of a large ongoing spectroscopic JWST program, I have obtained the first spectra that reveal early massive galaxies in unprecedented detail, along with a broader ‘context sample’ of several thousand galaxies. I have developed the necessary kinematic modelling tools to get dynamical masses of early galaxies, and have tailored stellar population models in place.We will robustly constrain the abundance of massive galaxies in the first billion years, their masses, structures and star formation histories, and AGN duty cycle. We will embed these results in the context of cosmological galaxy formation models. We can then determine how galaxy assembly took place in dark matter haloes, constrain the star formation efficiency, and answer how star formation was regulated in early galaxies. This proposal presents a breakthrough in our understanding of the first billion years, setting a new benchmark for galaxy formation models.