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Unravelling the physics of multiphase AGN winds through emission line tracers

Richings, Alexander J and Faucher-Giguère, Claude-André and Stern, Jonathan (2021) 'Unravelling the physics of multiphase AGN winds through emission line tracers.', Monthly Notices of the Royal Astronomical Society, 503 (2). pp. 1568-1585.


Observations of emission lines in active galactic nuclei (AGNs) often find fast (∼1000 km s−1) outflows extending to kiloparsec scales, seen in ionized, neutral atomic and molecular gas. In this work we present radiative transfer calculations of emission lines in hydrodynamic simulations of AGN outflows driven by a hot wind bubble, including non-equilibrium chemistry, to explore how these lines trace the physical properties of the multiphase outflow. We find that the hot bubble compresses the line-emitting gas, resulting in higher pressures than in the ambient interstellar medium or that would be produced by the AGN radiation pressure. This implies that observed emission line ratios such as [O IV]25μm / [Ne II]12μm⁠, [Ne V]14μm / [Ne II]12μm⁠, and [N III]57μm / [N II]122μm constrain the presence of the bubble and hence the outflow driving mechanism. However, the line-emitting gas is under-pressurized compared to the hot bubble itself, and much of the line emission arises from gas that is out of pressure, thermal and/or chemical equilibrium. Our results thus suggest that assuming equilibrium conditions, as commonly done in AGN line emission models, is not justified if a hot wind bubble is present. We also find that ≳50 per cent of the mass outflow rate, momentum flux, and kinetic energy flux of the outflow are traced by lines such as [N II]122μm and [Ne III]15μm (produced in the 104K phase) and [C II]158μm (produced in the transition from 104K to 100 K).

Item Type:Article
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Publisher statement:2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Date accepted:20 February 2021
Date deposited:16 September 2021
Date of first online publication:27 February 2021
Date first made open access:16 September 2021

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