Radiation-ionization hydrodynamic simulations of AGN line-driven winds lead to transient shielding and BAL/UFO signatures

Journal article

Disc winds from active galactic nuclei (AGNs) can be launched by radiation pressure acting on spectral lines. However, launching a line-driven wind in the X-ray-rich environment of AGNs is challenging, as the wind easily gets overionized. Previous simulations suggested that X-ray self-shielding could enable line-driving, though it remained unclear whether this relied on simplified treatments of radiation and ionization. Here, we revisit the X-ray shielding scenario using the first multifrequency multidirectional Monte Carlo radiative photoionization hydrodynamical simulations of AGN line-driven winds. We find that sustaining a steady wind with mass-loss rates of of the accretion rate requires an unrealistically weak X-ray flux (). For stronger X-ray emission (), self-shielding is only transient, leading to episodic ejections with mass-loss rates approaching the accretion rate. Our steady winds naturally produce FeLoBAL, HiBAL, and broad emission-line signatures, depending on the disc spectral energy distribution and the observer’s inclination. At moderate X-ray luminosities (), transient winds can generate short-lived BAL and ultra-fast outflow (UFO) features. At the highest X-ray luminosities (), the winds are too ionized to form BALs, but still produce UFOs. These results imply that additional physics is required to explain BAL outflows at realistic X-ray levels and to drive winds strong enough for AGN feedback. None the less, our simulations provide a new framework for interpreting the observed diversity of AGN outflow signatures with fully coupled radiation and dynamics.

Authors: Scepi, N; Knigge, C; Mosallanezhad, A; Long, KS; Matthews, JH

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Oxford University Press
• Journal: Monthly Notices of the Royal Astronomical Society
• Volume: 548
• Issue: 3
• Article number: stag592
• Publication date: 2026-04-30
• Acceptance date: 2026-03-20
• DOI: 10.1093/mnras/stag592
• EISSN: 1365-2966
• ISSN: 0035-8711
• Language: English
• Keywords: galaxies: active; quasars: absorption lines; hydrodynamics; stars: winds, outflows; accretion, accretion discs; radiative transfer