Morphology of Broad Emission Lines from Binary Supermassive Blackholes

Abstract

Accretion onto a blackhole can indue extreme radiation stimulating emission lines in nearby gases. In the deep gravitational well, these clouds move at speeds up to 10,000 km/s, and so broadens the emission lines via the relativistic Doppler Effect. These broad emission lines on spectra are the most prominent feature of AGNs. The region around central black hole, where such broad emission lines are produced, is known as the broad-line region (BLR). We developed a self-consistent model to describe both the geometry of the BLR and the dynamics of the gas clouds in the BLR. The motion of clouds is described by a series of Keplerian orbits. The spectral shift of emission line can be derived by the radial velocity at each point in Keplerian orbits. Assuming that the BLR is stable, that is, the structure and distribution of gas in BLR doesn’t change over a considerable period of time, the accumulated emission line profile can be constructed by simple addition of all Keplerian orbits in the BLR. By comparing the calculated profiles to spectra observed in SDSS (Sloan Digital Sky Survey), we found that our model can satisfactorily match most observed profiles. We attempted to review and summarize potential binary AGNs. We applied the model to the study of binary AGNs and their profiles of emission lines. We considered the velocity offset due to mutual Keplerian motion of black holes in binary and added their respective profiles to produce the final, resulting emission line profile from binary AGNs. Three kinds of emission line profiles are featured in our simulations: twin-peak structures where two strong broad-lines are present; sub-peak structures where a smaller but still visible emission line accompanies the dominant line; single broad emission peak but with significant velocity offset from the systematic redshift that is derived from the narrow emission-lines on the same spectrum. We then analyzed 1,348 AGN spectra with high S/N ratio in SDSS database, and selected 26 as candidate binary supermassive blackhole systems. These candidates are valuable for following-up observation, their binary nature could be confirmed by detecting the profile variation due to orbital motions.