Morphological evolution of galaxies to z ∼ 4

Galaxies have clearly evolved since the universe was 1 Gyr old, but methods to trace and quantify this evolution are still in their infancy. In this paper I demonstrate that with the careful use of a 'physical morphology' it is possible to determine quantitatively how the process of galaxy evolution is occurring out to z similar to 4. Using a system of parameters that traces star formation and galaxy interactions, I show how distinct galaxy populations at high-z can be identified in deep high-resolution optical and NIR images. These tools are also used to measure a potential merger fraction of galaxies from 0 < z < 4. If these methods axe reliable, as is suggested by a local galaxy calibration, the merger fraction of galaxies scales alpha (1+z)(2.1+/-0.5), peaks near z similar to 2, and declines thereafter. I also discuss how this system is likely part of the ultimate physical classification of galaxies.