The passbands are overlayed on three composite spectra from the distant cluster sample of galaxies. Given the extensive descriptions of the spectral indices available in the literature and the fact that we have concentrated on a statistical analysis of the spectra as opposed to a detailed investigation of the stellar populations, we give only a brief discussion of the individual indices used here. This index measures the only strong Ca I feature in this region of the spectrum. It is mostly sensitive to temperature and metallicity. They are mostly sensitive to temperature and metallicity, but also show gravitiy sensitivity for G stars.
Balmer lines are generally temperature sensitive in all stars.
Again, these indices show mostly temperature sensitivity, although there is significant metallicity sensitivity in this definition of the H index. Accordingly, the index is sometimes now denoted C 4668. It shows strong metallicity sensitivity, as well as gravity sensitivity. A principal component analysis transforms the observable axes, in this case, the spectral line strengths, which are necessarily interdependent, e.g. all partially temperature sensitive, into a new set of axes, or components, which are mutually orthogonal. This is accomplished by choosing each new axis to maximize the total variance along that axis.