Completion of a survey and detailed study of double-peaked emission lines in radio-loud active galactic nuclei

We report the completion of a survey of radio-loud active galactic nuclei (AGNs) begun in an earlier paper with the main goal of finding and studying broad, double-peaked Balmer lines. We present Halpha spectra of 13 more broad-lined objects, including three with double-peaked Halpha profiles. The final sample includes 106 radio-loud AGNs. In our final census 20% of objects have Halpha lines with double peaks or twin shoulders (the double-peaked emitters), and of these 60% (the disklike emitters) can be fitted quite well with a model attributing the emission to a circular, relativistic, Keplerian disk. In four objects where broad Hbeta and Mg II lines have been observed, we compare the profiles with models of photoionized accretion disks and find them to be in reasonable agreement. We reafirm the conclusion of Paper I that double-peaked emitters stand out among radio-loud AGNs on the basis of a number of additional properties that they possess: (1) an unusually large contribution of starlight to the optical continuum around Halpha, (2) unusually large equivalent widths of low-ionization lines ([O I] and [S II]), (3) unusually large [O I]/[O III] ratios, and (4) Balmer lines that are on average twice as broad as those of other radio-loud AGNs and preferentially redshifted. We consider and evaluate models for the origin of the lines, and we find accretion disk emission to be the most successful one because it can explain the double-peaked line profiles and it also offers an interpretation of the additional spectroscopic properties of these objects. We find the alternative suggestions (binary broad-line regions, bipolar outflows, anisotropically illuminated spherical broad-line regions) unsatisfactory because (1) they fail direct observational tests, (2) they cannot explain all of the unusual properties of disklike emitters self-consistently, or (3) in one case the physical foundations appear to be unsound. We suggest that double-peaked emitters and accretion-powered LINERs are the segment of the AGN population in which the accretion rate is considerably lower than the Eddington rate, with the consequence that the inner accretion disk takes the form of an ion torus and the wind that normally enshrouds the disk proper is absent.