A Review of Line and Continuum Correlations in AGNs

This review addresses the current observational status of the Baldwin Effect (BE), an empirical anti-correlation between emission line equivalent width and source luminosity for active galactic nuclei. We provide a brief history of the BE, and discuss selection effects and analysis methods relevant to its study. Existing research points to a number of detailed patterns in the BE, involving emission-line profiles and ionization, luminosity range, and radio properties, that provide clues to the underlying physics driving the correlation. We present a comparative analysis of the "global" BE for AGN ensembles and the "intrinsic" BE seen in variable Seyfert nuclei, and also examine similarities and differences in BEs reported in the optical/UV and X-ray bandpasses. A summary of measurements for quasars at high redshift (z > 4) suggests that the BE persists among these extreme objects. We close with an assessment of current needs and future opportunities for strengthening the observational picture of the BE.