The James Webb Space Telescope (JWST) is revolutionizing our view of the first billion years after the big bang, by enabling us to detect the primordial galaxies formed by the collapse of the Universe's first overdensities. Initial studies of the properties of these galaxies, partly carried out by our team, have revealed that their formation is still largely misunderstood and potentially in tension with the Lambda Cold Dark Matter (LCDM) model. Indeed, these studies have uncovered a potential excess of massive primordial galaxies, implying accelerated growth of these galaxies at star formation efficiencies well beyond the predictions of theoretical models. Before invoking radically different cosmological and galaxy evolution models, however, it is necessary to confirm these tensions, which are currently based only on highly uncertain measurements of the stellar mass of a few galaxies.
The aim of this thesis is to confirm or refute these tensions by accurately constraining, for the first time, the stellar mass of a large statistical sample of primordial galaxies. To do this, we will combine data from four JWST extragalactic surveys with an original statistical approach of image stacking, enabling us to obtain the average stellar mass of primordial galaxies that are otherwise too faint to be detected individually by the JWST in the critical mid-infrared window. This information, together with that obtained on their star-forming activity, will be decisive in understanding the growth of the Universe's first galaxies.