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We present results of Hubble Space Telescope NICMOS H-band imaging of 73 ofmost luminous (i.e., log[L_IR/L_0]>11.4) Infrared Galaxies (LIRGs) in the GreatObservatories All-sky LIRG Survey (GOALS). This dataset combinesmulti-wavelength imaging and spectroscopic data from space (Spitzer, HST,GALEX, and Chandra) and ground-based telescopes. In this paper we use thehigh-resolution near-infrared data to recover nuclear structure that isobscured by dust at optical wavelengths and measure the evolution in thisstructure along the merger sequence. A large fraction of all galaxies in oursample possess double nuclei (~63%) or show evidence for triple nuclei (~6%).Half of these double nuclei are not visible in the HST B-band images due todust obscuration. The majority of interacting LIRGs have remaining mergertimescales of 0.3 to 1.3 Gyrs, based on the projected nuclear separations andthe mass ratio of nuclei. We find that the bulge luminosity surface densityincreases significantly along the merger sequence (primarily due to a decreaseof the bulge radius), while the bulge luminosity shows a small increase towardslate merger stages. No significant increase of the bulge Sersic index is found.LIRGs that show no interaction features have on average a significantly largerbulge luminosity, suggesting that non merging LIRGs have larger bulge massesthan merging LIRGs. This may be related to the flux limited nature of thesample and the fact that mergers can significantly boost the IR luminosity ofotherwise low luminosity galaxies. We find that the projected nuclearseparation is significantly smaller for ULIRGs (median value of 1.2 kpc) thanfor LIRGs (mean value of 6.7 kpc), suggesting that the LIRG phase appearsearlier in mergers than the ULIRG phase.

The Nuclear Structure in Nearby Luminous Infrared Galaxies: HST NICMOS Imaging of the GOALS Sample