Spectroscopy of luminous infrared galaxies at 2 microns: 1. The ultraluminous galaxies (L[SUB]IR[/SUB] approximately greater than 10 [SUP]12[/SUP] solar luminosity)

We present high-quality spectra covering the K window at a resolving power of 340 for a sample of 13 ultraluminous (L_IR_ ~> 10^12^ L_sun_) infrared-selected galaxies, and line fluxes for a comparison sample of 24 lower luminosity galaxies. The 2 micron spectra of 10 of the ultraluminous galaxies are characterized by emission and absorption features commonly associated with stars and star formation; two others have the red power-law spectra and Brγ line widths of Seyfert 1 galaxies; the final galaxy has strong emission from hot dust. We have found no broad-line active nuclei not already known from optical observations, despite the fact that the extinction at 2 microns is 1/10 that at optical wavelengths; any putative Seyfert 1 nuclei must be deeply buried. Powerful continua and emission lines from H_2_ and Brγ are detected in all the ultraluminous galaxies. Comparing the H_2_ 1-0 S(1), Brγ, and 2 microns and far-infrared luminosities to those of the lower luminosity galaxies yields several major results. First, the dereddened Brγ emission, relative to the far-infrared luminosity, Is significantly depressed in the ultraluminous sample, when compared to the lower luminosity galaxies. Five of the ultraluminous galaxies have L_Brγ_/L_IR_ ratios lower than for any of the comparison objects. Second, the H_2_ 1-0 S(1) luminosity is proportional to the far-infrared luminosity, This suggests that the process producing the far-infrared luminosity is also responsible, directly or indirectly, for producing the excited H_2_, and that the H_2_ apparently comes from optically thin regions in both classes of objects. Third, eight of the 13 ultraluminous systems have lower 2 micron/far-infrared luminosity ratios than any of the lower luminosity galaxies, and five of these are the galaxies also deficient in Brγ. These three findings may be understood if the H_2_, Brγ, and 2 micron continua in the ultraluminous galaxies arise from spatially distinct regions, with the continuum and Brγ largely coming from volumes optically thick even at 2 microns, and obscured in such a fashion that the extinctions measured using optical spectroscopy do not properly measure the true optical depths. If this is the case, then even near-infrared spectroscopy may be unable to exclude the presence of undetected powerful active galactic nuclei in the ultraluminous galaxies.