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 Å to 8000 Å. 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 Å to 8000 Å) 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 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”, λrest ≈ 2000−4000 Å). A luminosity dependence is also present in the eigencoefficients and is related to the Baldwin Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260 Department of Physics and Astrophysics, University of Chicago, 5640 S. Ellis Ave, Chicago, IL 60637 Department of Physics and Astronomy, Johns Hopkins University, 3701 San Martin’s Drive, Baltimore, MD 21218 Princeton University Observatory, Peyton Hall, Princeton, NJ 08544 Department of Physics and Astronomy, Pennsylvania State University, University Park, PA 16802 Apache Point Observatory, 2001 Apache Point Road, P.O. Box 59, Sunspot, NM 88349-0059 Adler Planetarium and Astronomy Museum, 1300 Lake Shore Drive, Chicago IL 60605