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 since
observations indicating low formation efficiency and strong circum-glactic
presence of gas are persuasive. Energetic 'feedback' from massive stars and
accreting super-massive black holes - generally unresolved in cosmological
simulations - plays a major role for driving galactic outflows, which have been
shown to regulate many aspects of galaxy evolution. A surprisingly large
variety of plausible sub-resolution 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 multi-phase structure of the interstellar medium on the currently
unresolved few hundred parsecs 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, an accurate treatment of supernova explosions as well as the
non-thermal components of the interstellar medium like magnetic fields and
cosmic rays.
