The Lick AGN Monitoring Project 2011: Fe II Reverberation from the Outer Broad-Line Region

TL;DR

This study reports the first clear detection of Fe II reverberation in two Seyfert 1 galaxies, showing that Fe II emission responds to continuum changes with a longer lag than H-beta, indicating its origin in the outer broad-line region.

Contribution

First successful measurement of Fe II reverberation lag in active galactic nuclei, revealing its origin in the outer broad-line region, using spectral decomposition and cross-correlation techniques.

Findings

Fe II reverberation lag longer than H-beta lag

Fe II emission originates in outer broad-line region

Fe II variability correlates with continuum changes

Abstract

The prominent broad Fe II emission blends in the spectra of active galactic nuclei have been shown to vary in response to continuum variations, but past attempts to measure the reverberation lag time of the optical Fe II lines have met with only limited success. Here we report the detection of Fe II reverberation in two Seyfert 1 galaxies, NGC 4593 and Mrk 1511, based on data from a program carried out at Lick Observatory in Spring 2011. Light curves for emission lines including H-beta and Fe II were measured by applying a fitting routine to decompose the spectra into several continuum and emission-line components, and we use cross-correlation techniques to determine the reverberation lags of the emission lines relative to V-band light curves. In both cases the measured lag (t_cen) of Fe II is longer than that of H-beta, although the inferred lags are somewhat sensitive to the choice of Fe II template used in the fit. For spectral decompositions done using the Fe II template of Veron-Cetty et al. (2004), we find t_cen(Fe II)/t_cen(H-beta) = 1.9+-0.6 in NGC 4593 and 1.5+-0.3 in Mrk 1511. The detection of highly correlated variations between Fe II and continuum emission demonstrates that the Fe II emission in these galaxies originates in photoionized gas, located predominantly in the outer portion of the broad-line region.