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


Ishibashi, W; Fabian, A C (2018). Variations on a theme of AGN-driven outflows: luminosity evolution and ambient density distribution. Monthly Notices of the Royal Astronomical Society, 481(4):4522-4531.

Abstract

Galactic outflows are now commonly observed in starburst and active galactic nuclei (AGNs) 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 generalize 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 (⁠ζ=p˙/(L/c)⁠) and energy ratio (⁠ϵk=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 (ζ(r), εk(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.

Abstract

Galactic outflows are now commonly observed in starburst and active galactic nuclei (AGNs) 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 generalize 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 (⁠ζ=p˙/(L/c)⁠) and energy ratio (⁠ϵk=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 (ζ(r), εk(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.

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Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Physics Institute
Dewey Decimal Classification:530 Physics
Scopus Subject Areas:Physical Sciences > Astronomy and Astrophysics
Physical Sciences > Space and Planetary Science
Uncontrolled Keywords:Space and Planetary Science, Astronomy and Astrophysics
Language:English
Date:21 December 2018
Deposited On:16 Jan 2019 16:21
Last Modified:20 Jul 2024 01:37
Publisher:Oxford University Press
ISSN:0035-8711
OA Status:Green
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1093/mnras/sty2617
Project Information:
  • : FunderFP7
  • : Grant ID340442
  • : Project TitleFEEDBACK - ACCRETING BLACK HOLES AND COSMIC FEEDBACK
  • Content: Published Version