Broad emission lines are one of the dominant features of many AGN spectra. The broad-line region (BLR) plays a particularly important role in our understanding of AGNs by virtue of its proximity to the central source. The BLR potentially can provide a useful probe of the central source for at least two reasons. First, the bulk motions in the BLR are almost certainly determined by the central source, with gravity and radiation pressure competing. Second, the BLR reprocesses energy emitted by the continuum source at ionizing ultraviolet energies that cannot be observed directly, and thus the emission lines provide indirect information about this important part of the continuum.

Broad-Line Spectra

The broad-line spectra of AGNs show a great deal of diversity in terms of relative line strengths and profiles. The spectra shown in Figs. 1.1, 1.2, and 2.2 should be understood to be only more or less typical, as are the line strengths and equivalent widths of the stronger lines given in Table 1.1.

Widths of the broad lines show considerable differences from object to object. It is assumed that the lines are Doppler-broadened (§5.2), so line widths are almost always measured in velocity units, usually in terms of either (a) the full width at half maximum intensity (FWHM) or (b) the full width at zero intensity (FWZI); while the latter better reflects the true range of line-of-sight velocities, it is subject to uncertainties arising from ambiguities as to where the line actually reaches the continuum level and, in cases of especially high velocities, blending of the wings of different lines.