Near-IR HST Imaging of Ultraluminous IR Galaxies

A sample of 27 low-redshift Ultraluminous Infrared Galaxies (ULIRGs) has been imaged in the near-IR using the HST Near Infrared Camera and Multi-Object Spectrometer (NICMOS). These systems have also been imaged previously with the HST WFPC2 camera in the I-band. At least 85%\ of these systems show obvious morphological signs of current or past involvement in interaction events. The majority (67%) also contain multiple nuclei. The mean separation of the multiple cores is 5 kpc, with 28%\ having separations less than 2 kpc. Only 35%\ of the systems show either spectroscopic or morphological characteristics consistent with the presence of an active nucleus. The majority, therefore, appear to have collisionally-induced star bursts as their primary energy source. The average integrated H magnitudes of this sample is -24.3, which is only slightly above that of an L* galaxy. This indicates that these ULIRGs are not the result of the merging of two bright (i.e. >=L*) spiral galaxies, but are instead the product of two or more sub-L* galaxies. Only 1 out of the 49 nuclei identified in the sample has the properties of a bright QSO-like nucleus. On average, the brightest nuclei are 1.2 and 2.6 H magnitudes fainter than low- and high-luminosity QSOs, respectively. Nuclear starbursts with a constant star formation rate of 10--40 Msun per yr are consistent with the average H-band magnitudes of the first- and second-brightest nuclei in this sample of ULIRGs.