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

Abstract

We present a study of the morphology, kinematics and ionization structure of the extended emission-line regions in four intermediate-redshift (0.118<z<0.181) ultraluminous infrared galaxies, derived from ARGUS two-dimensional fibre spectroscopy. The gas kinematics in the hyperluminous system IRAS F20460+1925 lack coherent structure, with a full width at half-maximum (FWHM) >1000kms^-1 within 1arcsec of the nucleus, suggesting that any merger is well-advanced. Emission-line intensity ratios point to active galactic nucleus (AGN) photoionization for the excitation of this gas at the systemic velocity. An isolated blob ~8kpc from the nucleus with a much smaller velocity dispersion may lie in a structure similar to the photoionization cones seen in lower-luminosity objects. A second, spatially unresolved, narrow-line component is also present on nucleus, blueshifted by ~=990kms^-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 systemic velocity rotation curve is asymmetric in appearance, as a result either of the on-going merger or of nuclear dust obscuration. From a higher-resolution ISIS spectrum, we attribute the blue asymmetry in the narrow-line profiles to a spatially resolved nuclear outflow. Emission-line intensity ratios suggest shock+precursor ionization for the systemic component, consistent 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 ~1Gyr old starburst continuum. IRAS F01003-2238 has a dipolar velocity field and an Hii region emission-line spectrum with a strong blue continuum. After correction for intrinsic extinction, the latter can be reproduced with ~10⁷ O5 stars, sufficient to power the bolometric luminosity of the entire galaxy. We accommodate this diversity within the merger-induced evolutionary scenario for ultraluminous infrared galaxies: the merger status is assessed from the kinematics in a way which is consistent with morphological and colour information on the galaxies, or with the inferred ages of the young stellar populations and the dominance of the AGN.