It is a pleasure to review the observational basis for the Baldwin Effect (BE). Our meeting is timely for several reasons: the considerable progress in the field since the discovery of the BE, improved theoretical understanding of the emission-line region in quasars and active galactic nuclei (AGNs), the significant advances in multi-wavelength observations (from X-rays through to radio), and the prospects for future work that the new 8-10-m ground-based telescopes and space observatories will enable.
It is also appropriate that Jack Baldwin and Gary Ferland have taken the lead in organizing the meeting: Jack for his discovery and subsequent work on the BE and Gary for his work on developing theoretical and computing tools for understanding the emission-line regions of quasars.
We are glad to see Joe Wampler here. His work with Lloyd Robinson on developing the image tube scanner for the Lick 3-m telescope enabled Jack to make the observations that led to the discovery of the BE. The scanner was the first instrument that offered the requisite sensitivity, linearity, and broad wavelength coverage needed for the systematic study of emission lines in quasar spectra.
This paper is organized as follows. We give a brief history of the BE in the next section and a discussion of the selection and other effects that are important to its determination and interpretation. That is followed in Section 3 by a review of the current status of the BE, including different observational methods and analyses and the presence of the BE in other emission lines besides C IV. In Section 4 we consider more detailed luminosity effects, and in Section 5 we review the intrinsic BE, i.e., how the equivalent widths of emission lines change when the continuum level in a particular object varies. In Section 6 we discuss further systematics and phenomenology, such as which parts of the line profiles exhibit the BE and the X-ray BE. Then in Section 7 we review the BE in high-redshift objects and conclude in Section 8 with a summary and topics for future work.