Abstract

The broad emission lines of active galactic nuclei exhibit a wide variety of profile shapes and widths. A simple model is used to illustrate the possible significance of this diversity and to gain some insight into its physical origin.

Introduction: from igloos to Eiffel Towers

The broad emission lines are often the most prominent features in the optical-UV spectra of active galactic nuclei. Understanding how they are formed is therefore a key problem, not least because the emitting gas is thought to be closely associated with the fundamental energy source and so must be strongly influenced by its radiation field and the dynamical forces it produces. It is generally believed that the great widths of the line profiles arise mainly from large bulk velocities of the emitting gas. However, the structure and dynamics of the broad line region (BLR) are not understood in detail, although many possible models have been proposed (Section 2).

The shape of the emission line profile is one of the principle observational constraints on theories of the BLR. Initially, it was thought that the observations were adequately described by logarithmic profiles (Lλ∞–ln[Δλ]), as expected for radiation pressure acceleration (Blumenthal & Matthews 1975). However, as the quality and quantity of data have increased, it is becoming clear that other forms (e.g. power laws—van Groningen 1983; Penston et al. 1990) are often more appropriate.