Evidence for a Precessing Accretion Disk in the Nucleus of NGC 1097

We present new spectroscopic observations of the LINER (and now Seyfert 1)nucleus of NGC 1097, and discuss the evolution of its broad, double-peakedBalmer lines. When originally discovered in 1991, the red peak of thedouble-peaked H-alpha line was stronger than the blue, while by 1994 theH-alpha profile had become almost symmetric and the integrated line flux haddecreased to half its original value. Our new spectrum, taken in 1996, showsthat the broad, double-peaked lines have returned to almost their originalstrengths, the profiles of H-beta and H-alpha are identical to within errors,and the broad-line emitting region is unreddened. However, the profile of theBalmer lines is now such that the blue peak is stronger than the red, oppositeto the asymmetry observed in 1991. Various models are considered for theobserved behavior, all assuming that the emission lines originate in anaccretion disk. We present a refined version of the precessing, planar,elliptical accretion ring model proposed by Storchi-Bergmann et al. andEracleous et al. This model provides an acceptable fit to the line profiles. Wealso consider the possibility that the line profile evolution results from aprecessing warp in the disk, induced by irradiation from the center, and showthat the range of radii and precession time scales expected in this model areconsistent with the observations. The sudden appearance of the "disk-like"broad line profiles in NGC 1097 could have resulted from the formation of a newaccretion disk due to, for example, the tidal disruption of a star, or theillumination of a pre-existing disk by a transient ionizing source at thecenter of the disk.Comment: Accepted for publication in the Astrophysical Journal. TeX file with 5 postscript figures embeded using psfig.tex, 13 page