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Fe II Emission in AGN: The Role of Total and Gas-Phase Iron Abundance

Active galactic nuclei (AGN) have Fe II emission from the broad line region (BLR) that differs greatly in strength from object to object. We examine the role of the total and gas-phase iron abundance in determining Fe II strength. Using AGN spectra from the Sloan Digital Sky Survey (SDSS) in the redshift range of 0.2 < z < 0.35, we measure the Fe/Ne abundance of the narrow line region (NLR) using the [Fe VII]/[Ne V] line intensity ratio. We find no significant difference in the abundance of Fe relative to Ne in the NLR as a function of Fe II/Hbeta. However, the N II/S II ratio increases a by a factor of 2 with increasing Fe II strength. This indicates a trend in N/S abundance ratio, and by implication in the overall metallicity of the NLR gas, with increasing Fe II strength. We propose that the wide range of Fe II strength in AGN largely results from the selective depletion of Fe into grains in the low ionization portion of the BLR. Photoionization models show that the strength of the optical Fe II lines varies almost linearly with gas-phase Fe abundance, while the ultraviolet Fe II strength varies more weakly. Interstellar depletion of Fe can be as large as two orders of magnitude, sufficient to explain the wide range of optical Fe II strength in AGN. This picture is consistent with the similarity of the BLR radius to the dust sublimation radius and with indications of Fe II emitting gas flowing inwards from the dusty torus.