Using Near‐Infrared [Feii] Lines to Identify Active Galaxies

Open Access
Publisher Website
Google Scholar
Abstract
The ([Fe II] 1.644 μm)/Brγ and ([Fe II] 1.2567 μm)/Paβ ratios provide good separation of starburst and Seyfert galaxies; we show that ([Fe II] 1.644 μm)/Brγ is nearly as good a discriminant as the commonly used ([O I] λ6300)/Hα ratio. The behavior of both the ([O I] λ6300)/Hα and the ([Fe II] 1.644 μm)/Brγ ratios is readily understood if there is a progression in the proportion of shock excitation going from H II region through starburst and Seyfert galaxies to supernova remnants. We use the CLOUDY program to show that the behavior is also explained if the dominant excitation mechanism is photoionization. As a test of this technique for identifying excitation mechanisms, we have obtained infrared spectra of two LINER galaxies. NGC 1052, which is thought to be excited by a nuclear power-law UV source, is the prototype of LINERs, and NGC 3504 is classified as a weak-[O I] λ6300 LINER, better understood in terms of photoionization by O stars than by a nonstellar source. The placement of these galaxies on the ([Fe II] 1.644 μm)/Brγ versus ([O I] λ6300)/Hα diagram is in agreement with these previously determined characteristics. The ([Fe II] 1.644 μm)/Brγ and ([Fe II] 1.2567 μm)/Paβ ratios should be useful in classifying heavily obscured galaxies because the ratios (1) are constructed at wavelengths where extinction is relatively small and (2) involve lines over narrow spectral ranges.