Spectral classification of quasars in the Sloan Digital Sky Survey: Eigenspectra, redshift, and luminosity effects

We study 16,707 quasar spectra from the Sloan Digital Sky Survey (SDSS) ( an early version of the First Data Release; DR1) using the Karhunen- Loeve transform ( or principal components analysis). The redshifts of these quasars range from 0.08 to 5.41, the i-band absolute magnitudes from - 30 to - 22, and the resulting rest- frame wavelengths from 900 to 8000 Angstrom. The quasar eigenspectra of the full catalog reveal the following: first order - the mean spectrum; second order - a host-galaxy component; third order - the UV-optical continuum slope; fourth order - the correlations of Balmer emission lines. These four eigenspectra account for 82% of the total sample variance. Broad absorption features are found not to be confined in one particular order but to span a number of higher orders. We find that the spectral classification of quasars is redshift and luminosity dependent; as such there does not exist a compact set ( i.e., less than approximate to 10 modes) of eigenspectra ( covering 900 - 8000 Angstrom) that can describe most variations (i.e., greater than approximate to 95%) of the entire catalog. We therefore construct several sets of eigenspectra in different redshift and luminosity bins. From these eigenspectra we find that quasar spectra can be classified ( by the first two eigenspectra) into a sequence that is defined by a simple progression in the steepness of the slope of the continuum. We also find a dependence on redshift and luminosity in the eigencoefficients. The dominant redshift effect is a result of the evolution of the blended Fe II emission ( optical) and the Balmer continuum ( the small bump,'' lambda(rest) approximate to 2000 4000 Angstrom). A luminosity dependence is also present in the eigencoefficients and is related to the Baldwin effect - the decrease of the equivalent width of an emission line with luminosity, which is detected in Lyalpha, Si IV + O IV], C IV, He II, C III] and Mg II, while the effect in N v seems to be redshift dependent. If we restrict ourselves to the rest- wavelength regions 1150 - 2000 8 and 4000 - 5500 8, the eigenspectra constructed from the wavelength-selected SDSS spectra are found to agree with the principal components by Francis et al. and the well-known Eigenvector-1'' of Boroson & Green, respectively. ASCII formatted tables of the eigenspectra are available.