The Eisenstein et al. (2003) Sample

Finally, we get to the point of comparing our model predictions to galaxy data covering the full range of line indices considered in this paper. We are especially interested in contrasting the results based on blue and red indices. Until recently, high quality data for blue Lick indices were rare, if not entirely absent. The situation is changing quickly, as surveys of galaxies in the local and distant universe call for the need of a better understanding of the blue spectral region. A number of recent studies provided blue Lick index measurements based on moderate-to-high S/N spectra (e.g., Denicoló et al. 2005a,b, Rampazzo et al. 2005, Nelan et al. 2005, Sánchez-Blázquez et al. 2006a). We choose to analyze the stacked spectra from the Sloan Digital Sky Survey (SDSS) by Eisenstein et al. (2003). Our choice is chiefly motivated by the the fact that the stacked spectra are available publicly, so that we can perform our own index measurements and, most importantly, apply our own corrections for the effects of $\sigma$-broadening and Balmer line emission in-fill. Moreover, the stacked spectra have very high S/N ($>$ 400/${\rm\AA}$). Our procedure was described in Schiavon et al. (2006), but we briefly discuss the main aspects here.

The Eisenstein et al. (2003) sample consists of spectra of thousands of early-type galaxies from SDSS selected in terms of color (red) and morphology (bulge-dominated), with redshifts between $0.1 < z < 0.2$ (MAIN sample). The individual galaxy spectra sample a circular 3 region centered on each galaxy and the spectral resolution is $\sim$ 170 km s$^{-1}$ (FWHM). Galaxies were assigned to bins according to luminosity and environment, and all individual spectra within each bin were coadded so as to generate very high S/N spectra as a function of luminosity and environmental density. The main properties of the Eisenstein et al. sample are summarized in their Table 1. We choose to analyze their ``All'' sample, which refers to galaxies binned only by luminosity, regardless of environmental density. Because each luminosity bin includes galaxies of all environments, the number of coadded spectra per bin is always in excess of 2,500, so that the S/N of each stacked spectrum is extremely high. Our sample therefore consists of 4 spectra, one for each of the 4 luminosity bins in Eisenstein et al. (2003) sample (Table 28 and Table 1 in Eisenstein et al. ). We note that $M_r^\star \approx -20.8$ for this sample (Blanton et al. 2001), so that our results refer to bright galaxies only. The spectra were downloaded from D. Eisenstein's website (http://cmb.as.arizona.edu/$^\sim$eisenste/).