Galaxy Morphology and Classification

Central concentration of light

The galaxy classification system proposed by Morgan (1958,1959a), which is sometimes referred to as the Yerkes system, is a one-dimensional scheme based on central concentration of light. Morgan arranged galaxies in a sequence a–f–g–k, with objects of type a having the weakest central concentration of light and those of type k having the strongest central concentration. The Yerkes system also recognizes the intermediates types af, fg and gk. Abraham et al. (1996b) have shown that it is possible to measure a central concentration index C, determined from measurements of the intensity-weighted second-order moments of a galaxy image, which is closely related to the central concentration classification of the Yerkes system.

Galaxies with Morgan type a tend to have early-type (A) spectra, whereas galaxies of type k mostly exhibit late-type (K) integrated spectra. This linkage between morphology and spectral type shows that the dominant stellar population in centrally concentrated galaxies is old, whereas objects with a low central concentration of light tend to have a strong young population component. Probably this correlation between central concentration and integrated spectral type is largely due to the fact that regions of high gas density will usually collapse at earlier times than is the case for low density regions. This is so because the collapse time-scale τ∝(Gρ)−½. As we shall see in Chapter 11 a puzzling exception is provided by the dwarf spheroidal galaxies, most of which are dominated by an old stellar population, even though they are of low density.