2.2. Criteria for a Spectroscopic Classification of Galaxies
With the great improvements made in recent years in instrumentation and photographic emulsions, it now seems worth while to attempt a more precise spectroscopic classification of the integrated spectra of galaxies. This classification system has the following four properties:
1. It refers in general to two principal parts of a galaxy: (a) The system as a whole when there is little or no central concentration, and (b) the nuclear region when it dominates the system. In the first case, if there is any appreciable variation over the system, the spectral type corresponds to a weighted-mean sample of the main contributors to the total light.
2. A narrow interval in wavelength is used to obtain the spectral classification; in the present investigation it is that between 3850 and 4100. For a complete description of the composite spectrum of a galaxy, a spectral classification should be made for as many different wavelength regions as possible. In particular, classifications in the green, yellow, and red regions would add a great deal to the information obtainable from the blue-violet region. The crucial point, however, is that only similar spectral regions should be used when comparing spectroscopically the relative importance of stellar populations in different galaxies; a violet-region spectral type of one system should not be compared with a green-region type of another. Also, probably only similar parts of different systems should be compared.
3. An approach that is fundamentally two-dimensional is used for the spectral classification, and, as far as practicable, the Yerkes MK system was utilized. Some generalities concerning the classification of composite systems have already been outlined in connection with the spectral classification of globular clusters. 3
4. An attempt is made to interpret the spectral class-wavelength relationships with artificial or hypothetical H-R diagrams.