HST NICMOS imaging of z∼2, 24 μm-selected ultraluminous infrared galaxies

We present Hubble Space Telescope NICMOS H-band imaging of 33 ultraluminous infrared galaxies (ULIRGs) at z similar to 2 that were selected from the 24 mu m catalog of the Spitzer Extragalactic First Look Survey. The images reveal that at least 17 of the 33 objects are associated with interactions. Up to one-fifth of the sources in our sample could be minor mergers, whereas only two systems are merging binaries with luminosity ratio <= 3 : 1, which is characteristic of local ULIRGs. The rest-frame optical luminosities of the sources are of the order 10(10)-10(11) L(circle dot) and their effective radii range from 1.4 to 4.9 kpc. The most compact sources are either those with a strong active nucleus continuum or those with a heavy obscuration in the mid-infrared regime, as determined from Spitzer Infrared Spectrograph data. The luminosity of the 7.7 mu m feature produced by polycyclic aromatic hydrocarbon molecules varies significantly among compact systems, whereas it is typically large for extended systems. A bulge-to-disk decomposition performed for the six brightest (m(H) < 20) sources in our sample indicates that they are best fit by disklike profiles with small or negligible bulges, unlike the bulge-dominated remnants of local ULIRGs. Our results provide evidence that the interactions associated with ultraluminous infrared activity at z similar to 2 can differ from those at z similar to 0.