1999ApJ...525L..85M

Near-infrared spectra with a resolution of λ/Δλ~1100 in the rest-wavelength range of 1.8-2.2 µm have been obtained for a complete sample of 33 ultraluminous infrared galaxies (ULIRGs). Of the 33 objects observed, two show evidence of a central active galactic nucleus (AGN) through either a broad Paschen-α line or emission in the 1.963 µm fine structure of [Si VI]. In the median spectrum of the remaining 31 objects, the lines present are recombination lines of hydrogen and neutral helium, vibration-rotation lines of H₂, and [Fe II]. There is no indication of AGN activity in the median spectrum, either through broad atomic recombination lines or through high ionization lines. No trends in luminosity are apparent when subsets of the 31 non-AGN ULIRGs are binned by luminosity and median-combined. When secondary nuclei exist in ULIRGs, they typically have spectra very much like those seen in the primary nuclei.