Spectral classification of quasars in the sloan digital sky survey

Eigenspectra, redshift, and luminosity effects

C. W. Yip*, A. J. Connolly, D. E. Vanden Berk, Z. Ma, J. A. Frieman, M. Subbarao, A. S. Szalay, G. T. Richards, P. B. Hall, D. P. Schneider, A. M. Hopkins, J. Trump, J. Brinkmann

*Corresponding author for this work

Research output: Contribution to journalReview article

148 Citations (Scopus)


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 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 Å. 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 ≈10 modes) of eigenspectra (covering 900-8000 Å) that 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," λ rest≈2000-4000 A). 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 Lyα, 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 Å and 4000-5500 Å, 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.

Original languageEnglish
Pages (from-to)2603-2630
Number of pages28
JournalAstronomical Journal
Issue number6
Publication statusPublished - Dec 2004
Externally publishedYes


  • Methods: statistical
  • Quasars: general
  • Surveys
  • Techniques: spectroscopic Online material: Color figures

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    Yip, C. W., Connolly, A. J., Vanden Berk, D. E., Ma, Z., Frieman, J. A., Subbarao, M., ... Brinkmann, J. (2004). Spectral classification of quasars in the sloan digital sky survey: Eigenspectra, redshift, and luminosity effects. Astronomical Journal, 128(6), 2603-2630. https://doi.org/10.1086/425626