Astronomy Colloquium

I describe what can be learned about galaxy formation theories from studying how active galactic nuclei (AGN) occupy galaxies. In simulations, AGN play the role of quenching and preventing further star formation in the most massive galaxies, preventing them from becoming even higher stellar mass than they are. The cosmological simulations require subgrid physics to describe both star formation processes as well as black hole growth and feedback, both of which are complicated, nonlinear processes occurring at AU scales, within simulations with at best about 10 to 100 pc resolution. Among the major cosmological simulation codes, there are many different implementations of this subgrid physics that all lead to predictions of the stellar mass function and star formation rates of galaxies that agree with observations, and this agreement is taken very seriously as validation of the basic picture. These different implementations nevertheless lead to quite different predictions for how AGN populate galaxies as a function of mass and star formation rate, predictions which, in contrast to those regarding stellar properties of galaxies, are almost never compared against observations. I describe a research program to perform these tests, which requires a major revision of the statistics of AGN demographics in local galaxies, measurements which in many cases have been performed previously without regard to the selection effects of AGN in galaxies, which have dramatic effects. Although there are many methodological difficulties in interpreting the AGN predictions from galaxy formation simulations, we are finding that these simulations basically never show agreement with observations in this respect.