Quantifying the AGN-driven outflows in ULIRGs (QUADROS) III: Measurements of the radii and kinetic powers of 8 near-nuclear outflows

TL;DR

This study measures the sizes, densities, and kinetic powers of near-nuclear outflows in 8 ULIRGs, revealing modest outflow rates and powers with significant variability, contributing to understanding AGN feedback in galaxy evolution.

Contribution

First detailed measurements of radii and kinetic powers of outflows in ULIRGs using optical spectroscopy, expanding knowledge of AGN-driven feedback mechanisms.

Findings

Outflow regions are compact (0.08-1.5 kpc) and have high electron densities.

Mass outflow rates are modest, median around 2 solar masses per year.

Kinetic powers are a small fraction (median 0.3%) of AGN bolometric luminosity.

Abstract

As part of the QUADROS project to quantify the impact of AGN-driven outflows in rapidly evolving galaxies in the local universe, we present observations of 8 nearby ULIRGs ($0.04 < z < 0.2$) taken with the ISIS spectrograph on the \textit{William Herschel Telescope} (WHT), and also summarise the results of the project as a whole. Consistent with \cite{Rose2018}, we find that the outflow regions are compact (0.08 < $R_{[OIII]}$ < 1.5 kpc), and the electron densities measured using the [SII], [OII] trans-auroral emission-line ratios are relatively high (2.5 < log n$_{e}$(cm$^{-3}$) < 4.5, median log n$_{e}$(cm$^{-3})\sim 3.1$). Many of the outflow regions are also significantly reddened (median E(B-V) $\sim$ 0.5). Assuming that the de-projected outflow velocities are represented by the 5$\rm^{th}$ percentile velocities ($v_{05}$) of the broad, blueshifted components of [OIII]$\lambda$5007, we calculate relatively modest mass outflow rates ($0.1 < \dot{M} < 20$ M$_{\odot}$ yr$^{-1}$, median $\dot{M}$ $\sim$ 2 M$_{\odot}$ 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 \%, median $\dot{F}$ $\sim$ 0.3 \%). 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 disk 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.