3.7. Double-peaked Line Profiles
Broad emission lines are characteristic of the spectra of AGNs. If these lines arise in a disk, one expects them to exhibit the double-peaked profile structure that is commonly seen in the optical/UV emission lines from accretion disks in cataclysmic variables (see, e.g., Marsh & Horne 1988). Yet, only a fraction of radio-loud AGNs, and a handful of radio-quiet AGNs, show such lines (Eracleous & Halpern 1994; Storchi-Bergmann et al. 1997). This has been raised as a problem for accretion disk models of AGNs (Kinney 1994). However, it is also worth noting that Livio & Xu (1997) have shown that the double-peaked lines cannot be produced by two line-emitting jets, thus leaving the accretion disk as the only viable site for the production of such lines.
In accretion disk models the optical/UV continuum is generated from disks whose extent is 10-1000 Rg. If the optical/UV continuum is generated by an accretion disk, it is tempting also to hypothesize that the broad emission line spectrum is due to the disk. Collin-Souffrin (1987) showed that if a disk is illuminated by an external X-ray source, then the disk could contribute toward the emission-line spectrum, but the emission lines were from the outer parts of the disk at ~ 1000 Rg, and broad emission lines were not observed from the inner parts of the disk (Dumont & Collin-Souffrin 1990a, 1990b). Models by Chen, Halpern, & Filippenko (1989) and Chen & Halpern (1989) also effectively illuminate the outer parts of the disk to produce the low-ionization lines (Mg II and Balmer lines). In all the models, although the emission lines may be related to the disk phenomenon, the lines are formed at relatively large distances from the massive black hole, and we are interested in the inner parts of the accretion disk that predominantly contribute to the optical/UV continuum. Therefore in this paper we do not discuss broad emission line profiles further. Ultimately, the ionizing continuum that is generated by the inner disk drives the emission lines, and in a realistic model both the lines as well as the continuum will have to be explained. For a review on emission-line profiles from disks, we refer the reader to Eracleous (1998).
Before leaving this subject entirely, it is perhaps worthwhile pointing out that the Fe K line observed by ASCA in many Seyfert galaxies does occasionally have a double-peaked shape (see, e.g., Tanaka et al. 1995). We have discussed the Fe K line in Section 3.4 since this line is expected to arise within 100 Rg.