NICMOS Imaging of Infrared-Luminous Galaxies

We present near-infrared images obtained with the Hubble Space Telescope NICMOS camera for a sample of nine luminous [LIGs: L_IR(8-1000 μm)>=10¹¹ L_solar] and 15 ultraluminous (ULIGS: L_IR>=10¹² L_solar) infrared galaxies. The sample includes representative systems classified as warm (f_25μm/f_60μm>0.2) and cold (f_25μm/f_60μm<=0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as H II (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 (10⁷-10⁸ yr), formed as a result of galactic interactions, and having masses much greater than those of Galactic globular clusters. In NGC 6090 and VV 114, 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 (2.2 μm FWHM<=200 pc) nucleus. Analysis of the near-infrared colors (i.e., m_1.1-1.6 vs. m_1.6-2.2) 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 additional active galactic nucleus-like near-infrared components in their nuclei. 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 dichotomy of the near-infrared properties of the warm and the cold galaxies are further supported by the light distributions: seven of the eight warm galaxies have unresolved nuclear emission that contributes significantly (i.e., >=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 μm luminosity of these galaxies. Finally, light profiles of nine of the 24 systems were fitted well by an r^1/4 law (and not so well by an exponential disk profile). 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.