The Broad-Line Region in Active Galactic Nuclei

Abstract

We review the basic observed and inferred properties of the broad emission-line region in AGNs, as well as the basics of the reverberation-mapping technique that can be used to determine the size and structure of the region. We argue that the current best evidence points to a multi-component line-emitting re- gion, with a disk-like structure, possibly an extension of the accretion disk itself, and a disk wind being strong candidates for the origin of the broad-line emission. Some 40 years after the discovery of quasars and 60 years after the publication of C.K. Seyfert's initial observations of high central surface brightness galax- ies, we are finally quite certain that active galactic nuclei (AGNs) are powered by accretion onto supermassive collapsed objects. While many important de- tails remain poorly understood, the black-hole/accretion-disk paradigm is now reasonably secure. In contrast, however, we still have no self-consistent models of the nuclear regions that produce (a) the broad emission lines that are so prominent in the UV/optical spectra of AGNs and (b) the strong ab- sorption features seen in the X-ray/UV spectra. Given the proximity of these regions to the central engine, it seems likely that these are some manifes- tation of the accretion process and related outflow; there is a good deal of empirical evidence that connects these spectral features to disk-related out- flows. Unfortunately, solid information about the broad-line region (BLR) is hard to come by: the BLR is spatially unresolved in even the nearest AGNs and the information in line profiles, sampling only one of six dimensions in phase space, is highly ambiguous. In this review, we will concentrate on a few things that we can infer about the nature of the BLR, and reverberation mapping, arguably the most promising technique for exploring the nature of the BLR. The scope of this review is limited to a few basic topics and represents, of course, the author's own highly biased personal view.