Average Spectra of Massive Galaxies in the Sloan Digital Sky Survey

We combine Sloan Digital Sky Survey spectra of 22,000 luminous, red,bulge-dominated galaxies to get high S/N average spectra in the rest-frameoptical and ultraviolet (2600A to 7000A). The average spectra of these massive,quiescent galaxies are early-type with weak emission lines and with absorptionlines indicating an apparent excess of alpha elements over solar abundanceratios. We make average spectra of subsamples selected by luminosity,environment and redshift. The average spectra are remarkable in theirsimilarity. What variations do exist in the average spectra as a function ofluminosity and environment are found to form a nearly one-parameter family inspectrum space. We present a high signal-to-noise ratio spectrum of thevariation. We measure the properties of the variation with a modified versionof the Lick index system and compare to model spectra from stellar populationsyntheses. The variation may be a combination of age and chemical abundancedifferences, but the conservative conclusion is that the quality of the dataconsiderably exceeds the current state of the models.Comment: Astrophysical Journal, in press. 21 pages, LaTeX, 11 figures. Spectra available electronically from D. Eisenstein and eventually from Ap