K-band spectroscopy of ultraluminous infrared galaxies: The 2 Jy sample

We present near-infrared spectroscopy for a complete sample of 33 ultraluminous infrared galaxies at a resolution of R approximate to 1000. Most of the wavelength range from 1.80-2.20 mum in the rest frame is covered, including the Pa alpha and Br gamma hydrogen recombination lines, and the molecular hydrogen vibration-rotation 1-0 S(1) and S(3) lines. Other species, such as He I, [Fe II], and [Si VI] appear in the spectra as well, in addition to a number of weaker molecular hydrogen lines. Nuclear extractions for each of the individual galaxies are presented here, along with spectra of secondary nuclei, where available. The Pa alpha emission is seen to be highly concentrated on the nuclei, typically with very little emission extending beyond a radius of 1 kpc. This survey was carried out primarily to search for signatures of active nuclei via velocity-broadened hydrogen recombination or the presence of the [Si VI] coronal line. These signatures are rare in the present sample, occurring in only two of the 33 galaxies. The extinction to the hydrogen recombination lines is investigated via the Pa alpha /Br gamma line ratio. It is found that visual extinctions to the nuclei in excess of 10 mag are relatively common among ULIRGs and that visual extinctions greater than 25 mag are necessary to conceal a QSO emitting half the total bolometric luminosity. The ionized hydrogen regions in many ULIRGs are sufficiently obscured that dust-enshrouded active galactic nuclei would remain hidden at 2 mum at the current level of sensitivity. The vibration-rotation lines of molecular hydrogen appear to be predominantly thermal in origin, with effective temperatures generally around 2200 K. The relative nuclear velocities between double nucleus ULIRGs are investigated, through which it is inferred that the maximum deprojected velocity difference is similar to 200 km s(-1). This figure is lower than the velocities predicted by physical models of strong interactions/mergers of large, gas-rich galaxies.