  We present results of Hubble Space Telescope NICMOS H-band imaging of 73 of
most luminous (i.e., log[L_IR/L_0]>11.4) Infrared Galaxies (LIRGs) in the Great
Observatories All-sky LIRG Survey (GOALS). This dataset combines
multi-wavelength imaging and spectroscopic data from space (Spitzer, HST,
GALEX, and Chandra) and ground-based telescopes. In this paper we use the
high-resolution near-infrared data to recover nuclear structure that is
obscured by dust at optical wavelengths and measure the evolution in this
structure along the merger sequence. A large fraction of all galaxies in our
sample 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 to
dust obscuration. The majority of interacting LIRGs have remaining merger
timescales of 0.3 to 1.3 Gyrs, based on the projected nuclear separations and
the mass ratio of nuclei. We find that the bulge luminosity surface density
increases significantly along the merger sequence (primarily due to a decrease
of the bulge radius), while the bulge luminosity shows a small increase towards
late merger stages. No significant increase of the bulge Sersic index is found.
LIRGs that show no interaction features have on average a significantly larger
bulge luminosity, suggesting that non merging LIRGs have larger bulge masses
than merging LIRGs. This may be related to the flux limited nature of the
sample and the fact that mergers can significantly boost the IR luminosity of
otherwise low luminosity galaxies. We find that the projected nuclear
separation is significantly smaller for ULIRGs (median value of 1.2 kpc) than
for LIRGs (mean value of 6.7 kpc), suggesting that the LIRG phase appears
earlier in mergers than the ULIRG phase.
