Quasars from SDSS: Beyond Eigenvector 1

In 1992, Boroson & Green (BG92) discovered the so-called eigenvector 1; AGN exhibit an anticorrelation between [O III] λ5007Å and optical Fe II emission, coupled with the width of broad-line Hβ, such that stronger [O III] corresponds to broader Hβ and weaker Fe II. They showed (see also Boroson 2000) that these relationships are related to black hole mass or Eddington accretion ratio, and thus to the central engine. Surprisingly, the physics at sub-parsec scales is correlated with narrow line emission arising up to hundreds of parsecs away.

We revisit this fundamental relationship using the large numbers of quasar spectra provided by the Sloan Digital Sky Survey. Our sample includes 9508 spectroscopically-identified QSOs from Data Release 5 with a redshift range of 0.1 < z < 0.53, on which we perform spectral principal component analysis (SPCA). We find a subset of objects with extremely large [O III]/Hβ flux ratios that behave independent of Hβ linewidth. These objects drive nonlinear correlations among the data, and mask the correlation of FWHM of Hβ for the rest of the data set. However, these objects comprise less than a few percent of the entire data set, and thus are rare enough that they did not drive the relationships seen in previous, smaller samples such as BG92. These strong [O III]/Hβ objects do not seem to differ from the data set significantly in other spectral properties, such as luminosity, redshift, line shapes, and continuum slope. We conclude that they are not a special class of objects, just an extreme end of the normal quasar distribution.

Our conclusions demonstrate that for a more representative sample, the relationship between the broad- and narrow-line regions is more complex than suggested by early eigenvector analysis. The implications for accretion physics and AGN feedback are discussed.