Hydrodynamical Simulations of the Galaxy Population: Enduring Successes and Outstanding Challenges

We review the progress in modeling the galaxy population in hydrodynamical simulations of the 3CDM cosmogony. State-of-the-art simulations now broadly reproduce the observed spatial clustering of galaxies; the distributions of key characteristics, such as mass, size, and SFR; and scaling relations connecting diverse properties to mass. Such improvements engender confidence in the insight drawn from simulations. Many important outcomes, however, particularly the properties of circumgalactic gas, are sensitive to the details of the subgrid models used to approximate the macroscopic effects of unresolved physics, such as feedback processes. We compare the outcomes of leading simulation suites with observations, and with each other, to identify the enduring successes they have cultivated and the outstanding challenges to be tackled with the next generation of models. Our key conclusions include the following: Realistic galaxies can be reproduced by calibrating the ill-constrained parameters of subgrid feedback models. Feedback is dominated by stars and black holes in low-mass and high-mass galaxies, respectively. Adjusting or disabling the processes implemented in simulations can elucidate their impact on observables, but outcomes can be degenerate. Similar galaxy populations can emerge in simulations with dissimilar feedback implementations. However, these models generally predict markedly different gas flow rates into, and out of, galaxies and their halos. CGM observations are thus a promising means of breaking this degeneracy and guiding the development of new feedback models.