On the Feedback Efficiency of Active Galactic Nuclei

TL;DR

This study uses radiation-hydrodynamical simulations to analyze the feedback efficiencies of AGN-driven outflows, revealing lower energy feedback than previously assumed, with implications for black hole growth.

Contribution

First detailed simulation-based analysis of AGN feedback efficiencies on large scales, highlighting lower energy feedback and its impact on black hole evolution.

Findings

Maximum momentum feedback efficiency ~10^-2

Maximum mass feedback efficiency ~10^-1

Energy feedback efficiencies are much lower (~10^-5 to 10^-4) than 0.05

Abstract

(Abridged) We measure and analyze the energy, momentum, and mass feedback efficiencies due to radiation from AGN in relatively large scale outflows. Our measurements are based on the axisymmetric and time-dependent radiation-hydrodynamical simulations recently presented in Kurosawa & Proga. These simulations follow dynamics of gas under the influences of the AGN radiation and the gravity of the central 10^8 Msun black hole on scales from ~0.01 to ~10pc. We compare our Mdot_a-rho_o relation with that predicted by the Bondi accretion model. For high luminosities comparable to the Eddington limit, the power-law fit (Mdot_a ~ rho_o^q) to our models yields q=~0.5 instead of q=1.0 which is predicted by the Bondi model. This difference is caused by the outflows which are important for the overall mass budget at high luminosities. The maximum momentum and mass feedback efficiencies found in our models are ~10^-2 and ~10^-1, respectively. However, the outflows are much less important energetically: their thermal and kinetic powers in units of the radiative luminosity are ~10^-5 and ~10^-4, respectively. The efficiencies do not increase monotonically with the accretion luminosity but rather peak around the Eddington limit beyond which a steady state disk-wind-like solution exists. Our energy feedback efficiencies are significantly lower than 0.05, which is required in some cosmological and galaxy merger simulations. The low feedback efficiencies found here could have significant implications on the mass growth of super massive black holes in the early universe. We stress however that we have not considered the innermost parts of the accretion and outflow where radiation and matter interact most strongly. The feedback from this region could have efficiencies significantly above the low values found here.