Mapping the gas kinematics and ionization structure of four ultraluminous IRAS galaxies

(Abridged) We analyse the morphology, kinematics and ionization structure of the extended emission-line regions in four ULIRGs at 0.118<z<0.181, using ARGUS integral-field spectroscopy. The gas kinematics in the hyperluminous system IRAS F20460+1925 lack coherent structure, with FWHM>1000 km/s within 1" of the nucleus; an isolated blob ~8kpc from the nucleus with a much lower FWHM may lie in a photoionization cone. Line ratios suggest AGN-photoionization for the systemic gas, and shock+precursor ionization for a second, spatially-unresolved, nuclear component blueshifted by 990 km/s. The gas in IRAS F23060+0505 is in dipolar rotation but with an asymmetric rotation curve and an increased FWHM on one side; an asymmetry to the narrow-line profiles is attributed to a spatially-resolved outflow, and shock+precursor ionization is dominant. At lower luminosity, IRAS F01217+0122 has a featureless velocity field with a high FWHM, an AGN-like emission-line spectrum and a ~1 Gyr old stellar population. IRAS F01003-2238 has a dipolar velocity field and an HII-region emission-line spectrum with a strong blue continuum which can be reproduced with ~10^7 O5 stars, sufficient to account for the bolometric luminosity of the entire galaxy. This diversity is accommodated within the merger-driven evolutionary scenario for ULIRGs: the merger status is assessed from the kinematics in a way which is consistent with the colour and morphology of the galaxy, or with the inferred age of the stellar population and the dominance of the AGN.