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 s tarburst galaxies is studied. We carry out numerical calculations for photo ionization and shock heating, and compare the results with the observationa l 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] st rength. We propose that the [Fe II] strength depends primarily on the ioniz ation structure. The [Fe II] emission is excited by electron collisions in a zone of partially ionized hydrogen, which is extensive when the gas is he ated 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, an d 6000 K in shock heating. The [Fe II] emission of a Seyfert galaxy originates in the narrow-line regi on, which is heated by X-rays from the nucleus and by shocks due to the nuc lear 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 star burst galaxy originates in the supernova-driven shocks. We find that 0.2% o f the shock energy is transferred into the [Fe II] a(6)D(9/2)-a(4)D(7/2), l ine at 1.257 mu m.