The 1 micron FeII lines of the Seyfert galaxy I Zw 1

Infrared spectrophotometric observations from 0.8 to 2.5 mu m are presented for the narrow-line, type 1 Seyfert galaxy I Zn: 1. The data clearly revea l Fe II lambda 9997, lambda 10501, lambda 10863, and lambda 11126-the so-ca lled "1 mu m Fe II lines." These features are by far the strongest Fe II tr ansitions in the entire 0.8-2.5 mu m spectral region and, relative to the h ydrogen lines, are equal or stronger in I Zw 1 than in any of the Galactic sources in which they have been detected previously. The 1 mu m Fe II lines , which share a common upper term, are probably emitted by the broad line r egions of many active galaxies but have escaped identification because of b lending with nearby features of hydrogen and helium. A mechanism for their excitation through fluorescence by Ly alpha has been suggested previously i n the literature, but the crucial cascade lines that feed the upper term in this process are not seen in I Zw 1. The low energy of the upper term indi cates that the 1 mu m Fe II lines are collisionally excited.