Composite spectra of the red-sequence galaxies in each sub-structure (table 2). The flux densities of each spectrum are normalized at 4000 Å, and then stacked with an equal weight, i.e., we do not use a weight of |$1/\sigma ^2$| for stacking, because we prefer not to be biased towards bright targets. The pale region around the spectrum shows the |$1\sigma$| uncertainty. The color-coding in the spectra is the same as in figure 5. In this plot, the spectra are shifted arbitrarily towards the vertical direction to avoid overlapping each other. The wavelength shown in the horizontal axis is in rest frame. The left-hand panel (a) shows the spectra at the whole wavelength range of 3670–4900 Å, while the right-hand panel (b) shows the spectra at the limited range of 3670–4200 Å that are important for the spectral indices. In panel (b), the bars and pale-color regions show the wavelength ranges for each spectral index. (Color online)