The Relationship between Stellar Light Distributions of Galaxies and Their Formation Histories

Amajor problem in extragalactic astronomy is the inability to distinguish in a robust, physical, and model-independent way how galaxy populations are physically related to each other and to their formation histories. A similar, but distinct, and also long-standing question is whether the structural appearances of galaxies, as seen through their stellar light distributions, contain enough physical information to offer this classification. We argue through the use of 240 images of nearby galaxies that three model-independent parameters measured on a single galaxy image reveal its major ongoing and past formation modes and can be used as a robust classification system. These parameters quantitatively measure: the concentration (C), asymmetry (A), and clumpiness (S) of a galaxy’s stellar light distribution. When combined into a three-dimensional ‘ ‘ CAS ’ ’ volume all major classes of galaxies in various phases of evolution are cleanly distinguished. We argue that these three parameters correlate with important modes of galaxy evolution: star formation and major merging activity. This is argued through the strong correlation of H equivalent width and broadband colors with the clumpiness parameter S, the uniquely large asymmetries of 66 galaxies under-going mergers, and the correlation of bulge to total light ratios, and stellar masses, with the concentration index. As an obvious goal is to use this system at high redshifts to trace evolution, we demonstrate that these parameters can be measured, within a reasonable and quantifiable uncertainty with available data out to z 3 using theHubble Space TelescopeGOODSACS andHubble Deep Field images