Variations on a theme of AGN-driven outflows: luminosity evolution and ambient density distribution

TL;DR

This paper models AGN-driven galactic outflows considering luminosity evolution and ambient density, showing how radiation trapping and past AGN activity influence outflow energetics and can explain observed high-energy outflows.

Contribution

It extends previous models by including time-dependent AGN luminosity and ambient density effects, clarifying their roles in outflow energetics and evolution.

Findings

High outflow energetics can result from radiation trapping or AGN luminosity decay.

Decaying AGN luminosity alone cannot explain high energetics in expanding shells.

Radial profiles of outflow energetics can distinguish between trapping and decay scenarios.

Abstract

Galactic outflows are now commonly observed in starburst and active galactic nuclei (AGN) host galaxies. Yet, there is no clear consensus on their physical driving mechanism(s). We have previously shown that AGN radiative feedback, driven by radiation pressure on dust, can account for the observed dynamics and energetics of galactic outflows, provided that radiation trapping is taken into account. Here we generalise our model results by explicitly considering the temporal evolution of the central AGN luminosity, and the shell mass evolution in different ambient density distributions. In the case of fixed-mass shells, the high observed values of the momentum ratio ($\zeta = \dot{p}/(L/c)$) and energy ratio ($\epsilon_k = \dot{E}_{k}/L$) may be attributed to either radiation trapping or AGN luminosity decay. In contrast, for expanding shells sweeping up mass from the surrounding environment, a decay in AGN luminosity cannot account for the observed high energetics, and radiation trapping is necessarily required. Indeed, strong radiation trapping, e.g. due to high dust-to-gas ratios, can considerably boost the outflow energetics. We obtain a distinct radial dependence for the outflow energetics ($\zeta(r)$, $\epsilon_k(r)$) in the case of radiation trapping and luminosity decay, which may help discriminate between the two scenarios. In this framework, the recently discovered `fossil' outflows, with anomalously high values of the energetics, may be interpreted as relics of past AGN activity. The observed outflow properties may therefore provide useful constraints on the past history of AGN activity and/or the physical conditions of the outflow launch region.