2008ApJ...673...69K

We present a reverberation analysis of the strong, variable optical Fe II emission bands in the spectrum of Ark 120, a low-redshift AGN which is one of the best candidates for such a study. On timescales of several years the Fe II line strengths follow the variations in the continuum strength. However, we are unable to measure a clear reverberation lag time for these Fe II lines on any timescale. This is due to the very broad and flat-topped nature of the Fe II cross-correlation functions, as compared to the Hβ response, which is much more sharply localized in time. Although there is some suggestion in the light curve of a 300 day response time, our statistical analysis does not pick up such a feature. We conclude that the optical Fe II emission does not come from a photoionization-powered region similar in size to the Hβ-emitting region, but we cannot say for sure where it does come from. Our results are generally consistent either with emission from a photoionized region several times larger than the Hβ zone, or with emission from gas heated by some other means, perhaps responding only indirectly to the continuum variations.