A physical model for active galactic nuclei with double-peaked broad emission lines

Abstract

The double-peaked broad emission lines are usually thought to be linked to accretion disks, however, the local viscous heating in the line-emitting disk portion is usually insufficient for the observed double-peaked broad-line luminosity in most sources. Our calculations show that only a small fraction (< 2.3 per cent) of the radiation from the RIAF in the inner region of the disk can photo-ionize the line-emitting disk portion, because the solid angle of the outer disk portion subtended to the inner region of the RIAF is too small. We propose that only those AGNs with sufficient matter above the disk (slowly moving jets or outflows) can scatter enough photons radiated from the inner disk region to the outer line-emitting disk portion. Our model predicts a power-law r-dependent line emissivity with an index ~2.5, which is consistent with β~2-3 required by the model fittings for double-peaked line profiles. Using a sample of radio-loud double-peaked line emitters, we show that the outer disk regions can be efficiently illuminated by the photons scattered from the electron-positron jets with γ_j<2. It is consistent with the fact that no double-peaked emission line is present in strong radio quasars with relativistic jets. For radio-quiet counterparts, slow outflows with Thomson scattering depth ~0.2 can scatter sufficient photons to the line-emitting regions. This model can therefore solve the energy budget problem for double-peaked line emitters.