Galaxies and stars: a love story (with feedback)

Galaxies are complicated systems, but stars remind us that apparent complexity can sometimes be organized by physical principles into tight relations. For galaxies, the stellar–to–halo mass relation should play a similar role, encapsulating how efficiently dark matter halos turn baryons into stars. What physics enforces this relation, and why do very different simulations reproduce it? A promising picture, consistent with our new understanding of the interaction between hot and cold flows, emphasizes regulation through a critical energy balance between supernova heating and halo gas cooling, rather than the simple removal of cold gas. Following this idea leads us back to some surprising stellar observations and the need to look for help from the stars. For the most massive galaxies, black hole accretion enters the story, providing more evidence for this energy regulation picture but also more surprises. Making real progress on an organizing framework for galaxies may require tighter links between that eternal triangle: galaxy formation, stellar theory, and black hole physics.