Optical and Near‐Infrared Imaging of theIRAS1 Jy Sample of Ultraluminous Infrared Galaxies. II. The Analysis

The present paper discusses the results from an analysis of the imagespresented in Paper I (astro-ph/0207373) supplemented with new spectroscopicdata obtained at Keck. All but one object in the 1-Jy sample show signs of astrong tidal interaction/merger. Multiple mergers involving more than twogalaxies are seen in no more than 5 of the 118 (< 5%) systems. None of the 1-Jysources is in the first-approach stage of the interaction, and most (56%) ofthem harbor a single disturbed nucleus and are therefore in the later stages ofa merger. Seyfert galaxies (especially those of type 1), warm ULIGs(f_{25}/f_{60} > 0.2) and the more luminous systems (> 10^{12.5} L_sun) allshow a strong tendency to be advanced mergers with a single nucleus. Ananalysis of the surface brightness profiles of the host galaxies insingle-nucleus sources reveals that about 73% of the R and K' surfacebrightness profiles are fit adequately by an elliptical-like R^{1/4}-law. Theseelliptical-like 1-Jy systems have luminosities, half-light radii, and R-bandaxial ratio distribution that are similar to those of normal (inactive)intermediate-luminosity ellipticals and follow with some scatter the same mu_e- r_e relation. These elliptical-like hosts are most common among mergerremnants with Seyfert 1 nuclei (83%), Seyfert 2 optical characteristics (69%)or mid-infrared (ISO) AGN signatures (80%). In general, the results from thepresent study are consistent with the merger-driven evolutionary sequence``cool ULIGs --> warm ULIGs --> quasars,'' although there are many exceptions.(abridged)