ON the feedback efficiency of active galactic nuclei

Abstract

We measure and analyze the energy, momentum, and mass feedback efficiencies due to radiation from active galactic nuclei (AGNs) in relatively large-scale outflows (from ∼ 0.01 to ∼ 10pc). Our measurements are based on the two-dimensional (axisymmetric) and time-dependent radiation-hydrodynamical simulations recently presented in Kurosawa & Proga. In that paper, we studied outflows from a slowly rotating (sub-Keplerian) infalling gas driven by the energy and pressure of the radiation emitted by the AGNs. These simulations follow the dynamics of gas under the influence of the gravity of the central 108 M · black hole (BH) on scales from ∼ 0.01 to ∼ 10pc. They self-consistently couple the accretion luminosity with the mass inflow rate at the smallest radius (our proxy for the mass-accretion rate,Ma). Over 30 simulations have been performed to investigate how the results depend on the gas density at the outer radius, ρo. A key feature of these simulations is that the radiation field and consequently the gas dynamics are axisymmetric, but not spherically symmetric. Therefore, the gas inflow and outflow can occur at the same time. We compare our M a-ρo relation with that predicted by the Bondi accretion model. For high luminosities comparable to the Eddington limit, the power-law fit (Ma ∝ ρoq) 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: the thermal and kinetic powers in units of the radiative luminosity are ∼10-5 and ∼10-4, respectively. In addition, 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 BHs 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. © 2009 The American Astronomical Society. All rights reserved.