An objective classification scheme for QSO spectra

Abstract

The classification of QSO rest-frame optical and UV spectra is undertaken by means of a statistical principal component analysis (PCA) emphasizing the relation between QSO properties. The PCA is applied directly to a sample of QSO UV spectra from the Large Bright QSO Survey, and relations are studied between continuum, emission-line, and broad-absorption-line properties. Quantitative classifications are derived based on a limited number of principal components, and three components account for approximately 75 percent of the sample and describe: emission-line core strength, continuum slope, and broad absorption features. Broad-absorption-line QSOs appear as a natural subclass of the population with characteristic excess emissions, and continuum hardness correlates with the equivalent widths of C IV and Lyman-alpha emission lines. Quantitative properties are derived from the study that serve to classify QSO emissions ranging from broad low-equivalent-width lines to narrow high-equivalent-width lines.