Main-sequence of star-forming galaxies: New insights and perspectives on massive galaxy formation

IAP weekly specialised seminars / 7 November 2025

Benjamin Magnelli (Laboratoire CosmoStat, Laboratoire AIM, Irfu, CEA-Saclay (Gif-sur-Yvette, France))

The observed tight correlation between the star formation rate (SFR) and the stellar mass of star-forming galaxies (SFGs) is now, thanks to recent JWST observations, well constrained over the past 12 billion years of cosmic history. This so-called main sequence (MS), whose normalization steadily declines from z~6 to 0, is commonly interpreted as evidence that massive SFGs evolve primarily through a steady, long-lasting mode of star formation, likely sustained by cold gas accretion along the cosmic web. Over the last decade, numerous studies have explored, within this framework, the physical properties of SFGs both along and across the MS, establishing key scaling relations between, for example, stellar mass, gas content, and morphology in the SFR–stellar mass plane. In this talk, I will begin by reviewing past and recent observational evidence supporting this main-sequence paradigm and how it has shaped our understanding of the evolution of massive galaxies. I will then present how new observations, particularly from JWST and ALMA, allow us to move beyond this model, providing fresh insights into a surprisingly large population of already massive galaxies in the early Universe, as well as into the mechanisms driving the morphological transformation of massive SFGs as they evolve along the main sequence and transition to quiescence. I will conclude by highlighting future observational opportunities that will further probe the physical processes governing the evolution of massive galaxies across cosmic time.