Emission Line Diagnostics

Summary

This review focus on three major aspects of emission line diagnostics: Line intensities that are used to deduce the physical conditions in the gas, line profiles that are needed to study the gas dynamics, and line variability, that is used to obtain the gas distribution. Applications and examples are given for active galaxies and quasars. The status of research and the outstanding problems in each of these areas are discussed and new observational findings are shown. The more important developments of recent years are due to systematic, combined space and groundbased observations of individual objects (reverberation mapping) as well as studies of large samples of AGNs (e.g. the HST radio-loud sample).

Introduction and overview of active galactic nuclei

Observations of emission lines in photoionized nebulae provide important diagnostics of the line emitting gas in three different ways: Line intensities are used to derive the physical conditions in the gas. Density, temperature, optical depth etc. are all related to emission line ratios and absolute fluxes. Line profiles are used to investigate the gas dynamics and the velocity field. Finally, line variability, when correlated with flux variations of the photoionizing continuum, are used to measure the gas distribution and the size of the emission line region.

Active Galactic Nuclei (AGN) are situated in the center of otherwise normal galaxies and show strong emission lines superimposed on strong nonstellar continua.