Galaxies evolve over billions of years in part through the activity of star formation and their supermassive nuclear black holes, and also by mergers with other galaxies. Some features of galaxies, in particular the strong correlations found between the mass of the central black hole and properties like galaxy velocity structure or luminosity, imply a fundamental connection between the growth of the nuclear black hole and the assembly of stars on a global scale. Feedback of some kind is therefore expected to explain these tight correlations, and astronomers have been working to identify and study it. One prominent suggestion for feedback is the presence of warm outflowing gas, powered by new stars but which would deplete the galaxy of the raw material needed for making new stars, and/or for enhancing the black hole mass.