| Abstract
| - Context. Ultraluminous infrared galaxies (ULIRGs) are among the brightest objects in the local Universe. They are powered by strong star formation and/or an AGN. As a result of the merger process, they are also likely to be the progenitors of elliptical galaxies. The study of the structure and kinematics of samples of local ULIRGs is necessary to understand the physical processes that these galaxies undergo, and their implications for our understanding of similar types of galaxies at high redshift. Aims. The goal of the project is to analyze the structure, dust distribution, ionization state, and kinematics of a representative sample of 22 ULIRGs. The galaxies in the sample undergo different merger phases (they are evenly divided between pre- and post-coalescence systems) and ionization stages (27% H ii, 32% LINER, 18% Seyfert, and 23% mixed classifications) over a wide infrared luminosity range (11.8 ≤$L_{\rm IR}/L_{odot}$≤ 12.6), which also includes some galaxies of low luminosity. The main aims of this paper are to present the sample and discuss the structure of the stellar and ionized gas components. Methods. Our study relies on the use of integral field optical spectroscopy data obtained with the INTEGRAL instrument at the 4.2 m William Herschel Telescope. Results. The structure of the ionized gas as traced by different emission lines has been studied and compared with that of the stellar continuum. We find structural variations between the gaseous and the stellar components, with offsets in the emission peaks positions of up to about 8 kpc. The observed morphological differences are due to the spatial distribution of the ionization sources and the presence of dust. Young star formation (as traced by the H α emission) is present in all regions of the galaxies. However, for 64% of ULIRGs in an early interaction phase, the young star formation peak does not coincide with the stellar maxima. In contrast, galaxies undergoing advanced mergers have a H α peak that is located in their nuclear regions. In three of the studied ULIRGs (one pre- and two post-coalescents), hard ionizing photons traced by the [O iii] λ5007 line excite extra-nuclear nebulae out to distances of about 7 kpc. These regions do not show bright stellar emission, but are rather dominated by nebular emission. These galaxies have nuclei classified as Seyfert in the literature. Approximately 40% of the pre-coalescence ULIRGs exhibit shifts between the peaks of their red continuum and that local to the [O i] λ6300 line. However, some of these peaks are associated with the secondary stellar nucleus of the system. In contrast, the emission in post-coalescence ULIRGs is concentrated towards the nuclei. These results imply that evolution caused by a merger is ocurring in the ionized gas structure of ULIRGs.
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