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

We present a study of the morphology, kinematics and ionization structure o f the extended emission-line regions in four intermediate-redshift (0.118 < z < 0.181) ultraluminous infrared galaxies, derived from ARGUS two-dimensi onal fibre spectroscopy. The gas kinematics in the hyperluminous system IRAS F20460 + 1925 lack cohe rent structure, with a full width at half-maximum (FWHM) > 1000 km s(-1) wi thin 1 arcsec of the nucleus, suggesting that any merger is well-advanced. Emission-line intensity ratios point to active galactic nucleus (AGN) photo ionization for the excitation of this gas at the systemic velocity. An isol ated blob similar to 8 kpc from the nucleus with a much smaller velocity di spersion may lie in a structure similar to the photoionization cones seen i n lower-luminosity objects. A second, spatially unresolved, narrow-line com ponent is also present on nucleus, blueshifted by similar or equal to 990 k m s(-1) from the systemic and plausibly powered by photoionizing shocks. IRAS F23060 + 0505 has more ordered kinematics, with a region of increased FWHM coincident with the blue half of a dipolar velocity field. The systemi c velocity rotation curve is asymmetric in appearance, as a result either o f the on-going merger or of nuclear dust obscuration. From a higher-resolut ion ISIS spectrum, we attribute the blue asymmetry in the narrow-line profi les to a spatially resolved nuclear outflow. Emission-line intensity ratios suggest shock + precursor ionization for the systemic component, consisten t with the X-ray view of a heavily obscured AGN. The lower-luminosity objects IRAS F01217 + 0122 and F01003 - 2238 complete the sample. The former has a featureless velocity field with a high FWHM, a high-ionization AGN spectrum and a similar to 1 Gyr old starburst continuu m. IRAS F01003 - 2238 has a dipolar velocity field and an H II region emiss ion-line spectrum with a strong blue continuum. After correction for intrin sic extinction, the latter can be reproduced with similar to 10(7) O5 stars , sufficient to power the bolometric luminosity of the entire galaxy. We accommodate this diversity within the merger-induced evolutionary scenar io for ultraluminous infrared galaxies: the merger status is assessed from the kinematics in a way which is consistent with morphological and colour i nformation on the galaxies, or with the inferred ages of the young stellar populations and the dominance of the AGN.