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-Loève (KL) transform (or Principal Component Analysis, PCA). The redshifts of these quasars range from 0.08 to 5.41, the i-band absolute magnitudes from −30 to −22, and the resulting restframe wavelengths from 900 ˚A to 8000 ˚A. The quasar eigenspectra of the full catalog reveal the following: 1st order — the mean spectrum; 2nd order — a host-galaxy component; 3rd order — the UV-optical continuum slope; 4th 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 ≈ 10 modes) of eigenspectra (covering 900 ˚A to 8000 ˚A) which can describe most variations (i.e., greater than ≈ 95 %) of the entire catalog. We therefore construct several sets of eigenspectra in different redshift and luminosity bins. From these eigenspectra