Galaxy Form and Spectral-Type: A Physical Framework for Measuring Evolution

I outline a quantitative method for characterizing galaxies both by photometric ‘form’ and indices of spectral-type, applicable to both nearby and distant galaxies. Such a characterization provides insight on galaxy evolution because there are physical connections between galaxies’ stellar populations and their light distribution. “Normal” galaxies’ form-parameters (surface-brightness, image concentration and asymmetry) correlate well with spectral-index (color), which in turn correlates only weakly with scale (size or luminosity). Deviations from these normal relations also offer clues to the physical modes of galaxy formation and evolution. As an example, I contrast a puzzling, distant population of compact, but luminous, blue, star-forming galaxies to nearby samples. These distant sources appear to be associated with the bulk of the luminosity increase since z < 1. They have structural properties comparable to low-redshift populations, and photometric properties within the norm for nearby, actively star-forming galaxies. When combined, however, their photometric and structural properties appear to be highly unusual.