Quantifying the AGN-driven outflows in ULIRGs (QUADROS) – I: VLT/Xshooter observations of nine nearby objects

Abstract

Although now routinely incorporated into hydrodynamic simulations of galaxy\nevolution, the true importance of the feedback effect of the outflows driven by\nactive galactic nuclei (AGN) remains uncertain from an observational\nperspective. This is due to a lack of accurate information on the densities,\nradial scales and level of dust extinction of the outflow regions. Here we use\nthe unique capabilities of VLT/Xshooter to investigate the warm outflows in a\nrepresentative sample of 9 local (0.06 $<$ z $<$ 0.15) ULIRGs with AGN nuclei\nand, for the first time, accurately quantify the key outflow properties. We\nfind that the outflows are compact (0.05 $<$ R$_{[OIII]}$ $<$ 1.2 kpc),\nsignificantly reddened (median E(B-V)$\\sim$0.5 magnitudes), and have relatively\nhigh electron densities (3.4 $<$ log$_{10}$ n$_e$ (cm$^{-3}$) $<$ 4.8). It is\nnotable that the latter densities -- obtained using trans-auroral [SII] and\n[OII] emission-line ratios -- exceed those typically assumed for the warm,\nemission-line outflows in active galaxies, but are similar to those estimated\nfor broad and narrow absorption line outflow systems detected in some type 1\nAGN. Even if we make the most optimistic assumptions about the true\n(deprojected) outflow velocities, we find relatively modest mass outflow rates\n($0.07 < \\dot{M} < 11$ M$_{sol}$ yr$^{-1}$) and kinetic powers measured as a\nfraction of the AGN bolometric luminosities ($4\\times10^{-4} < \\dot{E}/L_{BOL}\n<1$%). Therefore, although warm, AGN-driven outflows have the potential to\nstrongly affect the star formation histories in the inner bulge regions ($r\n\\sim$ 1kpc) of nearby ULIRGs, we lack evidence that they have a significant\nimpact on the evolution of these rapidly evolving systems on larger scales.\n