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One of the proposed explanations for the broad, double-peaked Balmer emissionlines observed in the spectra of some active galactic nuclei (AGNs) is thatthey are associated with sub-parsec supermassive black hole (SMBH) binaries.Here, we test the binary broad-line region hypothesis through several decadesof monitoring of the velocity structure of double-peaked H-alpha emission linesin 13 low-redshift, mostly radio-loud AGNs. This is a much larger set ofobjects compared to an earlier test by Eracleous et al. (1997) and we use muchlonger time series for the three objects studied in that paper. Althoughsystematic changes in radial velocity can be traced in many of their lines,they are demonstrably not like those of a spectroscopic binary in a circularorbit. Any spectroscopic binary period must therefore be much longer than thespan of the monitoring (assuming a circular orbit), which in turn would requireblack hole masses that exceed by 1-2 orders of magnitude the values obtainedfor these objects using techniques such as reverberation mapping and stellarvelocity dispersion. Moreover, the response of the double-peaked Balmer lineprofiles to fluctuations of the ionizing continuum and the shape of theLy-alpha profiles are incompatible with a SMBH binary. The binary broad-lineregion hypothesis is therefore disfavored. Other processes evidently shapethese line profiles and cause the long-term velocity variations of the doublepeaks.

A Radial Velocity Test for Supermassive Black Hole Binaries as an Explanation for Broad, Double-Peaked Emission Lines in Active Galactic Nuclei