Abstract
Infrared spectrophotometric observations from 0.8 to 2.5 μm are presented for the narrow-line, type Seyfert galaxy I Zw 1. The data clearly reveal Fe II λ9997, λ10501, λ10863, and λ11126—the so-called 1 μm Fe II lines. These features are by far the strongest Fe II transitions in the entire 0.8-2.5 μm spectral region and, relative to the hydrogen lines, are equal or stronger in I Zw than in any of the Galactic sources in which they have been detected previously. The μm Fe II lines, which share a common upper term, are probably emitted by the broad line regions of many active galaxies but have escaped identification because of blending with nearby features of hydrogen and helium. A mechanism for their excitation through fluorescence by Lyα has been suggested previously in 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 indicates that the μm Fe II lines are collisionally excited.
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