Excitation mechanism of forbidden Fe II 1. 644 micron emission in Seyfert and starburst galaxies

The excitation mechanism of the forbidden Fe II emission in AGN and starbust nuclei (SBN) is studied by comparing the intensity of the forbidden Fe II 1.644 micron emission with those of low-ionization optical lines. A linear correlation is found between the line ratio of forbidden Fe II/Br-gamma and forbidden O I /H-alpha, while the correlation between forbidden Fe II/Br-gamma and forbidden N II/H-alpha is poor. The forbidden O I lines is radiated from hydrogen transition regions while the forbidden N II line comes from fully ionized regions. It is concluded that the forbidden Fe II emission is excited by electron collisions in extended transition regions. In SBN, the forbidden Fe II emission is due to shock heating caused by starburst winds rather than photoionization. The forbidden Fe II emission is stronger in Seyfert and composite galaxies than in pure starburst galaxies. 66 refs.