NICMOS Imaging of Infrared-Luminous Galaxies

We present near-infrared images obtained with the HST NICMOS camera for a sample of 9 luminous (LIGs: LIR(8 − 1000μm) ≥ 10 11 L⊙) and 15 ultra-luminous (ULIGS: LIR ≥ 10 12 L⊙) infrared galaxies. The sample includes representative systems classified as warm (f25μm/f60μm > 0.2) and cold (f25μm/f60μm ≤ 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as HII (i.e. starburst), QSO, Seyfert and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters. The colors and the luminosities of the observed clusters are consistent with them being young (10 yrs), formed as a result of galactic interactions and having masses much greater than those of Galactic globular. In NGC 6090 and VV114, they are preferentially situated along the area of overlap of the two galactic disks. With the exception of IR 17208-0018, all of the ULIGs have at least one compact (2.2μm FWHM ≤ 200 pc) nucleus. Analysis of the near-infrared colors (i.e., m1.1−1.6 vs. m1.6−2.2) derived from 1.1 ′′ diameter apertures suggests that the warm galaxies have near-infrared colors consistent with QSO+hot dust emission and the cold galaxies, as a group, have near-infrared colors consistent with reddened starlight. In addition, the cold ULIG UGC 5101 (and possibly three others) have near-infrared colors suggesting an additional AGN-like near-infrared component in the nucleus. In a 2 kpc-diameter aperture measurement, the global colors of all of the cold galaxies except UGC 5101 are consistent with starlight with a few magnitudes of visual extinction. The general