Qualitative comparison of our composite passive spectrum (black line) with the empirical templates quoted in the text that is representative of passive galaxies residing in different environments and redshift ranges. All spectra are normalized in the wavelength range 3950 Å < λ < 4050 Å and smoothed with a three pixel boxcar filter. Left: the K20 composite spectrum of distant early-type galaxies in the field is shown in red while the template computed by Kinney et al. (1996) for elliptical galaxies in groups at z ~ 0.008 is shown in magenta. It is evident that the passive galaxies of our sample are redder w.r.t. the K20 ones at similar redshift, but bluer than the ellipticals in groups in the local universe. Right: comparison between the average spectrum of our passive galaxies and the LRG template provided by Eisenstein et al. (2003) (in green) that was computed for z  ~ 0.4, M_g <  −21.8 galaxies. Although the two spectra were obtained by using different galaxy selection criteria and redshift ranges, they appear to be remarkably similar overall (except for a slightly higher flux in U band for the LRG spectrum).