Theoretical Challenges in Galaxy Formation

Numerical simulations have become a major tool for understanding galaxy formation and evolution. Over the decades the field has made significant progress. It is now possible to simulate the formation of individual galaxies and galaxy populations from well-defined initial conditions with realistic abundances and global properties. An essential component of the calculation is to correctly estimate the inflow to and outflow from forming galaxies because observations indicating low formation efficiency and strong circumgalactic presence of gas are persuasive. Energetic " feedback" from massive stars and accreting supermassive black holes-generally unresolved in cosmological simulations-plays a major role in driving galactic outflows, which have been shown to regulate many aspects of galaxy evolution. A surprisingly large variety of plausible subresolution models succeeds in this exercise. They capture the essential characteristics of the problem, i. e., outflows regulating galactic gas flows, but their predictive power is limited. In this review, we focus on one major challenge for galaxy formation theory: to understand the underlying physical processes that regulate the structure of the interstellar medium, star formation, and the driving of galactic outflows. This requires accurate physical models and numerical simulations, which can precisely describe the multiphase structure of the interstellar medium on the currently unresolved few hundred parsec scales of large-scale cosmological simulations. Such models ultimately require the full accounting for the dominant cooling and heating processes, the radiation and winds from massive stars and accreting black holes, and an accurate treatment of supernova explosions as well as the nonthermal components of the interstellar medium like magnetic fields and cosmic rays.