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

Abstract

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 δ < 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 Hα, [N ii], [S ii] and [O i] into individual components, and chart the Hα 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.

Shocked gas in IRAS F17207-0014: ISM collisions and outflows

“…A number of authors have examined optical emission lines for signs of outflows as well. Arribas & Colina (2003) found no evidence of outflows in their INTEGRAL observations (although they do suggest an inflowing component to the southwest); Westmoquette et al (2012) also find no evidence for outflows in their VIMOS data. However, fit multiple components to the emission lines in their optical long-slit spectra and produce measurements of velocity offset (∼-100 km s −1 from systemic) and velocity dispersion (∼100 km s −1 ) for the shocked components, which they sum together to produce an estimated outflow velocity of 200 km s −1 ; they also use emission line ratios and shock models to infer shock velocities ranging from 50-600 km s −1 .…”

High-resolution imaging of the molecular outflows in two mergers: IRAS 17208-0014 and NGC 1614

“…In spite of previous negative evidence of an outflow in the ionized gas (Westmoquette et al 2012;Rupke & Veilleux 2013), Arribas et al (2014) report the detection of an outflowing component, based on very high signal-to-noise ratio Hα spectroscopy. The maximum velocities of the ionized outflow reach moderate values of up to v − v sys ∼ −300 km s −1 and the estimated outflow rate for the ionized gas is ∼16 M yr −1 (Colina, private communication).…”

X-shooter reveals powerful outflows in z ∼ 1.5 X-ray selected obscured quasi-stellar objects

“…Only very recently spatially resolved optical, far infrared and mm spectroscopic studies convincingly showed evidences for the existence of such processes in the form of the predicted energetic outflows in ULIRGs with Seyfert nuclei in the local Universe (e.g. Feruglio et al 2010;Fischer et al 2010;Rupke & Veilleux 2011;Villar-Martín et al 2011;Rodríguez-Zaurín et al 2013;Zhang et al 2011;Westmoquette et al 2012;Rupke & Veilleux 2013;Harrison et al 2014;McElroy et al 2014; see also Elvis 2000 for a discussion on the ubiquitous presence of winds in the AGN structure).…”