Double-peaked broad line emission from the LINER nucleus of NGC 1097

Abstract

We report the recent appearance of a very broad component in the Hα and Hβ emission lines of the weakly active nucleus of the Sersic-Pastoriza galaxy NGC 1097. Subtraction of the contribution of the stellar population from our spectra reveals that the FWZI of the broad component is about 21,000 km s-ˡ and that its profile is double-peaked. The data also suggest the presence of a blue, featureless continuum in the nucleus. The Hα/Hβ flux ratio indicates that the broad-line region (BLR) is not significantly reddened. The broad component was first observed in Ho: in 1991 November 2, and confirmed 11 months later. The Hα profile and flux did not change in this time interval. Comparison with previously published spectral data indicates that the broad lines have only recently appeared. Together with the relatively high X-ray luminosity and the compact nuclear radio source, our results characterize the presence of a Seyfert 1 nucleus in a galaxy which had previously shown only LINER characteristics. The simplest interpretation is that obscuring material along our line of sight to the nucleus has recently cleared, permitting a direct view of the active nucleus. We discuss two possible structures for the broad line region-biconical outflow and an accretion disk-that could give rise to the observed profile. We also speculate on the relationship between the broad line region and the previously known optical jets, which extend to tens of kpc from the nucleus.