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Excitation Mechanism of Near-Infrared [Fe II] Emission in Seyfert and Starburst Galaxies

The excitation mechanism of near-infrared [Fe II] emission in Seyfert and starburst galaxies is studied. We carry out numerical calculations for photoionization and shock heating, and compare the results with the observational data. The gas-phase abundance of iron is found to be always low. This means that destruction of iron-bearing grains is insufficient to affect the [Fe II] strength. We propose that the [Fe II] strength depends primarily on the ionization structure. The [Fe II] emission is excited by electron collisions in a zone of partially ionized hydrogen, which is extensive when the gas is heated by X-rays or by a shock. These two processes are discriminated by the electron temperature of the [Fe II] region: 8000 K in heating by X-rays, and 6000 K in shock heating. The [Fe II] emission of a Seyfert galaxy originates in the narrow-line region, which is heated by X-rays from the nucleus and by shocks due to the nuclear outflow. From the [Fe II] temperature, we propose that the heating by X-rays is more important. On the other hand, the [Fe II] emission of a starburst galaxy originates in the supernova-driven shocks. We find that 0.2% of the shock energy is transferred into the [Fe II] a 6 D 9/2-a 4 D 7/2 line at 1.257 μm.