Quantifying the AGN-driven outflows in ULIRGs (QUADROS) III: measurements of the radii and kinetic powers of eight near-nuclear outflows

Abstract

As part of the Quantifying ULIRG AGN-driven Outflows (QUADROS) project to quantify the impact of active galactic nuclei (AGN)-driven outflows in rapidly evolving galaxies in the local Universe, we present observations of eight nearby ultraluminous infrared galaxies (ULIRGs, 0.04 <z < 0.2) taken with the Intermediate-dispersion Spectrograph and Imaging System on the William Herschel Telescope (WHT), and also summarize the results of the project as a whole. Consistent with Rose et al. (2018), we find that the outflow regions are compact (0.08 < R_[O III] < 1.5 kpc), and the electron densities measured using the [S II], [O II] trans-auroral emission-line ratios are relatively high (2.5 < log n_e (cm^-3) < 4.5, median log n_e (cm^-3) ∼ 3.1). Many of the outflow regions are also significantly reddened (median E(B - V) ∼ 0.5). Assuming that the deprojected outflow velocities are represented by the fifth percentile velocities (v₀₅) of the broad, blueshifted components of [O III] λ5007, we calculate relatively modest mass outflow rates (0.1 < \dot{M} < 20 M_⊙ yr^-1, median \dot{M} ∼ 2 M_⊙ yr^-1), and find kinetic powers as a fraction of the AGN bolometric luminosity (\dot{F}=\dot{E}/L_bol) in the range 0.02 < \dot{F} < 3 per cent (median \dot{F} ∼ 0.3 per cent). The latter estimates are in line with the predictions of multi-stage outflow models, or single-stage models in which only a modest fraction of the initial kinetic power of the inner disc winds is transferred to the larger scale outflows. Considering the QUADROS sample as a whole, we find no clear evidence for correlations between the properties of the outflows and the bolometric luminosities of the AGN, albeit based on a sample that covers a relatively small range in L_bol. Overall, our results suggest that there is a significant intrinsic scatter in outflow properties of ULIRGs for a given AGN luminosity.