NICMOS imaging of infrared-luminous galaxies

We present near-infrared images obtained with the Hubble Space Telescope NI CMOS camera for a sample of nine luminous [LIGs: L-IR(8-1000 mu m) greater than or equal to 10(11) L.] and 15 ultraluminous (ULIGS: L-IR greater than or equal to 10(12) L.) infrared galaxies. The sample includes representativ e systems classified as warm (f(25 mu m)/f(60 mu m) > 0.2) and cold (f(25 m u m)/f(60 mu m) less than or equal to 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as H II (i.e., starburs t), QSO, Seyfert, and LINER. The morphologies of the sample galaxies are di verse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are s een 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(7)-10(8) yr), formed as a result of galactic interactions, and having masses much g reater than those of Galactic globular clusters. In NGC 6090 and VV 114, th ey are preferentially situated along the area of overlap of the two galacti c disks. With the exception of IR 17208-0018, all of the ULIGs have at leas t one compact (2.2 mu m FWHM less than or equal to 200 pc) nucleus. Analysi s of the near-infrared colors (i.e., m(1.1-1.6) vs. m(1.6-2.2)) derived fro m 1." 1 diameter apertures suggests that the warm galaxies have near-infrar ed colors consistent with QSO+hot dust emission and the cold galaxies, as a group, have near-infrared colors consistent with reddened starlight. In ad dition, the cold ULIG UGC 5101 (and possibly three others) have near-infrar ed colors suggesting additional active galactic nucleus-like near-infrared components in their nuclei. In a 2 kpc diameter aperture measurement, the g lobal colors of all of the cold galaxies except UGC 5101 are consistent wit h starlight with a few magnitudes of visual extinction. The general dichoto my of the near-infrared properties of the warm and the cold galaxies are fu rther supported by the light distributions: seven of the eight warm galaxie s have unresolved nuclear emission that contributes significantly (i.e., gr eater than or equal to 30%-40%) to the total near-infrared luminosity. The smooth, more extended light observed in all of the galaxies is most likely composed of giant and supergiant stars, but evidence at longer wavelengths suggests that these stars contribute little to the high 8-1000 mu m luminos ity of these galaxies. Finally, light profiles of nine of the 24 systems we re fitted well by an r(1/4) law (and not so well by an exponential disk pro file). Whether these star systems eventually become massive central bulges or giant elliptical galaxies will depend on how efficiently the present ISM is converted into stars.