Why we need wide-field imaging from space to understand galaxy formation [review article]

Space-based imaging with the Hubble Space Telescope has demonstrated the many advantages of viewing galaxies from outside the Earth's atmosphere. Hubble has shown us the amazing complexity of galaxies in the nearby and high redshift universe in exquisite detail. However, because of the limited areas surveyed, the sample sizes remain relatively small. In order to go beyond "butterfly collecting" and unravel the underlying physical processes that shape galaxy formation, we need samples that span a broad range of environment as well as cosmic time, and that are large enough to sub-divide in many different ways. I discuss three broad science topics that would benefit from wide-field space-based surveys: (1) measuring the stellar mass assembly history as a function of morphological type and environment and directly constraining the galaxy merger rate; (2) understanding the connection between the processes that regulate star formation on sub-galactic scales and the overall global trends in star formation activity; (3) understanding the relationship between "normal" galaxies and AGN, and the processes that regulate black hole feeding and growth. I present quantitative predictions, based on a simple halo occupation model, for the field sizes needed to reduce the error due to field-to-field variance to less than 10%. For many of the populations of interest, this will require fields of ∼10-100 square degrees.