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

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 selfshielded flows and can cover a large portion of the ionizing flux-density (φ H -n H ) 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 f V 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 (LOC) 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.

Broad He i 1.08-µm absorption from the obscurer in the active galaxy NGC 5548

“…The wind, initially launched perpendicular to the disc plane, bends under the radiation pressure and then propagates asymptotically roughly at an angle of 45 • with respect to the symmetry axis (see e.g. Elvis 2000;Proga, Stone & Kallman 2000;Matthews et al 2020). If so, we observe the source under the conditions characteristic for broad-absorption line (BAL) quasars, partially looking through the flow.…”

Placing LOFAR-detected quasars in C iv emission space: implications for winds, jets and star formation

“…Alternatively, the probability of producing a disc wind could increase with blueshift such that there are more high blueshift objects with detectable wind-related radio emission. Disc wind models for BAL quasars are capable of producing strong UV emission lines (Murray et al 1995;Matthews et al 2016Matthews et al , 2020, but, although there have been successful attempts to model C IV blueshifts (e.g. Chajet & Hall 2013;Yong et al 2017), we are not aware of a full radiative transfer and photoionization treatment of their formation.…”

Optical line spectra of tidal disruption events from reprocessing in optically thick outflows

“…Hamann & Koesterke 1998 ;Hamann et al 2013 ). Microclumping in PYTHON has been discussed in previous work by Matthews et al ( 2016Matthews et al ( , 2020 and, in the context of the problem at hand by P20. Within the micro-clumping framework, clumps are assumed to be optically thin, smaller than all rele v ant lengthscales and embedded in a vacuum.…”