Abstract
Stars form from cold molecular gas in galaxies, at the heart of dark matter halos. However, not all galaxies form their stars in the same way : in every epoch, we can distinguish between galaxies that are actively forming stars, the main-sequence galaxies, and galaxies that form very few stars, if any at all. Most star formation in the Universe took place in main-sequence galaxies. And while our own Milky Way forms just a few solar masses of stars per year, ten billion years ago, main-sequence galaxies formed ten times as many. What do these galaxies, where most star formation took place, look like ? How do they evolve over time ? To what extent does star formation depend on the amount of molecular gas available ? Why might they stop forming stars at some point in their history ?
