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Stratified disc wind models for the AGN broad-line region: ultraviolet, optical, and X-ray properties

Stratified disc wind models for the AGN broad-line region: ultraviolet, optical, and X-ray properties
Stratified disc wind models for the AGN broad-line region: ultraviolet, optical, and X-ray properties
The origin, geometry, and kinematics of the broad-line region (BLR) gas in quasars and active galactic nuclei (AGN) are uncertain. We demonstrate that clumpy biconical disc winds illuminated by an AGN continuum can produce BLR-like spectra. We first use a simple toy model to illustrate that disc winds make quite good BLR candidates, because they are self-shielded flows and can cover a large portion of the ionizing flux-density (ϕH-nH) plane. We then conduct Monte Carlo radiative transfer and photoionization calculations, which fully account for self-shielding and multiple scattering in a non-spherical geometry. The emergent model spectra show broad emission lines with equivalent widths and line ratios comparable to those observed in AGN, provided that the wind has a volume filling factor of fV ≲ 0.1. Similar emission line spectra are produced for a variety of wind geometries (polar or equatorial) and for launch radii that differ by an order of magnitude. The line emission arises almost exclusively from plasma travelling below the escape velocity, implying that ‘failed winds’ are important BLR candidates. The behaviour of a line-emitting wind (and possibly any ‘smooth flow’ BLR model) is similar to that of the locally optimally emitting cloud model originally proposed by Baldwin et al. (1995), except that the gradients in ionization state and temperature are large-scale and continuous, rather than within or between distinct clouds. Our models also produce UV absorption lines and X-ray absorption features, and the stratified ionization structure can partially explain the different classes of broad absorption line quasars.
0035-8711
Matthews, James H.
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Knigge, Christian
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Higginbottom, Nick
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Long, Knox S.
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Sim, Stuart A.
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Mangham, Samuel W.
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Parkinson, Edward J.
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Hewitt, Henrietta A.
066fadf1-2841-4b8e-a09e-90299c472777
Matthews, James H.
211abe1f-c722-4a7c-9d85-275b18edf681
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Higginbottom, Nick
602bc39e-24c2-47fe-b39d-450681ec47af
Long, Knox S.
e749f6fb-96dc-489b-aa2c-86151f4ff579
Sim, Stuart A.
95ef9134-b7d7-4fbb-af7a-b00f8d5c8b65
Mangham, Samuel W.
2d24c74d-e3c7-4cb9-8a89-944298890259
Parkinson, Edward J.
14ddf718-9da1-4c49-ae5a-4d9a1985c8eb
Hewitt, Henrietta A.
066fadf1-2841-4b8e-a09e-90299c472777

Matthews, James H., Knigge, Christian, Higginbottom, Nick, Long, Knox S., Sim, Stuart A., Mangham, Samuel W., Parkinson, Edward J. and Hewitt, Henrietta A. (2020) Stratified disc wind models for the AGN broad-line region: ultraviolet, optical, and X-ray properties. Monthly Notices of the Royal Astronomical Society. (doi:10.1093/mnras/staa136).

Record type: Article

Abstract

The origin, geometry, and kinematics of the broad-line region (BLR) gas in quasars and active galactic nuclei (AGN) are uncertain. We demonstrate that clumpy biconical disc winds illuminated by an AGN continuum can produce BLR-like spectra. We first use a simple toy model to illustrate that disc winds make quite good BLR candidates, because they are self-shielded flows and can cover a large portion of the ionizing flux-density (ϕH-nH) plane. We then conduct Monte Carlo radiative transfer and photoionization calculations, which fully account for self-shielding and multiple scattering in a non-spherical geometry. The emergent model spectra show broad emission lines with equivalent widths and line ratios comparable to those observed in AGN, provided that the wind has a volume filling factor of fV ≲ 0.1. Similar emission line spectra are produced for a variety of wind geometries (polar or equatorial) and for launch radii that differ by an order of magnitude. The line emission arises almost exclusively from plasma travelling below the escape velocity, implying that ‘failed winds’ are important BLR candidates. The behaviour of a line-emitting wind (and possibly any ‘smooth flow’ BLR model) is similar to that of the locally optimally emitting cloud model originally proposed by Baldwin et al. (1995), except that the gradients in ionization state and temperature are large-scale and continuous, rather than within or between distinct clouds. Our models also produce UV absorption lines and X-ray absorption features, and the stratified ionization structure can partially explain the different classes of broad absorption line quasars.

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Stratified disc wind models for the AGN broad-line region: ultraviolet, optical, and X-ray properties - Accepted Manuscript
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Accepted/In Press date: 9 January 2020
e-pub ahead of print date: 16 January 2020
Additional Information: arXiv is AM

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Local EPrints ID: 439450
URI: http://eprints.soton.ac.uk/id/eprint/439450
ISSN: 0035-8711
PURE UUID: 4278ff52-db63-44b9-a421-65c7632ce2ad

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Date deposited: 23 Apr 2020 16:33
Last modified: 06 Oct 2020 20:16

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Contributors

Author: James H. Matthews
Author: Nick Higginbottom
Author: Knox S. Long
Author: Stuart A. Sim
Author: Samuel W. Mangham
Author: Edward J. Parkinson
Author: Henrietta A. Hewitt

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