Until recently, galaxies have been studied back to when the universe was only 5-7% of its current age, yet little is known about the processes that shape their early evolution and reionize most of the neutral gas in the universe by that time. New observations of the James Webb Space Telescope (JWST) and Atacama Large Millimetre Array move this timeline to only 2% and reveal that galaxies appear to be more evolved in the early universe than previously thought. The most distant galaxies appear to produce more light or form sooner than expected from the physics of star formation in local galaxies or than predicted by the cosmological model. By extension, models tuned in the later universe suggest that the early galaxies are also more massive than expected. In addition to that, the nature of the strong ionising radiation in some early galaxies is still unknown and can be tied to distinct types of massive stars or active galactic nuclei. The wealth of new information challenges our understanding of galaxies as complex systems and suggests new alleys for modern research.

This work addresses the problems of excessive stellar light and mass by investigating possible differences in the physics of star formation. The socalled mass-to-light ratio is one of the fundamental properties of galaxies that connects star formation and the total amount of galaxy light. Various case studies of old and young stellar systems in the Milky Way and elliptical galaxies in the late universe, pointed to possible over or under-production of certain types of stars compared to the standard models. The study here implements some of these changes in the galaxy models and tests them using a large sample of images of distant galaxies in multiple wavelengths. The findings show that active galaxies tend to form more massive stars in the past and extrapolating this effect can explain most of the unusual luminous objects observed with the JWST. Therefore, modifications to the fundamental assumptions about star formation may be required. In the future, the new galaxy models need to be tested on the observations of chemical abundances in the earliest galaxies that can be sensitive to distinct types of stars. Finally, this work also presents possible evidence of star formation in one of the most extreme environments in the universe – active galactic nuclei, which, if confirmed, can become one of the testing grounds for extreme star formation conditions in the future.