Periodic variations in the first and second moments of broad Balmer emission lines from central accretion disks

Abstract

Broad emission line regions (BLRs) lying in central accretion disks are widely accepted to explain the unique double-peaked broad emission lines in active galactic nuclei (double-peaked BLAGNs). Accepting the accretion disk origin, we theoretically simulated and determined the periodic variations in the central wavelength λ_{0 (the first moment) and line width σ (the second moment) of double-peaked broad emission lines. Furthermore, through theoretically simulated periodicities of Tλ0 and Tσ for variations in λ0 and σ, a periodicity ratio Rfs of Tλ0 to Tσ of about 2 was applied to support the hypothesis that the spiral arms are preferred in BLRs in central accretion disks. Then, we determined the periodic variations in λ0 and σ and showed them in the known double-peaked BLAGN NGC 1097. This led to the parameter Rfs ∼ 2, which can be applied as a clue to support the hypothesis that the spiral arm structure in disk-like BLRs in the central accretion disk is the most compelling interpretation of the variability in the double-peaked broad Hα in NGC 1097. The results provide clean criteria for testing the accretion disk origins of double-peaked broad emission lines in AGN.}