Spatially resolved observations of warm ionized gas and feedback in local ULIRGs

We present Very Large Telescope (VLT)/VIsible MultiObject Spectrograph integral field unit (VIMOS-IFU) emission-line spectroscopy of a volume-limited sample of 18 southern ultraluminous infrared galaxies (ULIRGs) selected with z < 0.09 and d < 10. By covering a wide range of ULIRG types, including many systems that have received very little previous attention, this data set provides an important set of templates for comparison with high-redshift galaxies. We employed an automated Gaussian line-fitting programme to decompose the emission-line profiles of Ha, [N ii], [S ii] and [O i] into individual components, and chart the Ha kinematics, and the ionized gas excitations and densities. 11 out of 18 of our galaxies show evidence for outflowing warm ionized gas with speeds between 500 and a few 1000 km s-1, with the fastest outflows associated with systems that contain an active galactic nucleus. Our spatially resolved spectroscopy has allowed us to map the outflows, and in some cases determine for the first time to which nucleus the wind is associated. In three of our targets we find line components with widths >2000 km s-1 over spatially extended regions in both the recombination and forbidden lines; in two of these three, they are associated with a known Sy2 nucleus. Eight galaxies have clear rotating gaseous discs, and for these we measure rotation velocities, virial masses, and calculate Toomre Q parameters. We find radial gradients in the emission-line ratios in a significant number of systems in our study. We attribute these gradients to changes in ionizing radiation field strength, most likely due to an increasing contribution of shocks with radius. We conclude with a detailed discussion of the results for each individual system, with reference to the existing literature. Our observations demonstrate that the complexity of the kinematics and gas properties in ULIRGs can only be disentangled with high sensitivity, spatially resolved IFU observations. Many of our targets are ideal candidates for future high spatial resolution follow-up observations.