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

Abstract

Disc winds from active galactic nuclei (AGN) can be launched by radiation pressure acting on spectral lines. However, launching a line-driven wind in the X-ray rich environment of AGN is challenging, as the wind easily gets over-ionized. 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 multi-frequency, multi-directional Monte-Carlo radiative photo-ionization hydrodynamical simulations of AGN line-driven winds. We find that sustaining a steady wind with mass-loss rates of $\approx20\%$ of the accretion rate requires an unrealistically weak X-ray flux ($α_{\rm OX}<-3$). For stronger X-ray emission ($-3<α_{\rm OX}<-1$), 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 ($α_{\rm OX}\sim-3$), transient winds can generate short-lived BAL and ultra-fast outflow (UFO) features. At the highest X-ray luminosities ($α_{\rm OX}\sim-1$), 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. Nonetheless, our simulations provide a new framework for interpreting the observed diversity of AGN outflow signatures with fully coupled radiation and dynamics.

Figures (12)

• Figure 1: Spectral energy distributions used as input in our simulations.
• Figure 2: Mass-loss rate in solar mass per year (solid lines) and kinetic luminosity in Eddington units (dotted lines) as a function of time in years for all of our simulations. The left column shows the simulations with the truncated disc SED while the right column shows the simulations with the full disc SED. From top to bottom the X-ray luminosity of the central source increases. The green lines show the times at which we make snapshots and spectra in the rest of the paper.
• Figure 3: Density (right) and velocity (left) maps of 6 selected simulations. The left column shows the simulations with the truncated disc SED while the right column shows the simulations with the full disc SED. From top to bottom the X-ray luminosity of the central source increases. On each colorplot a polar plot shows the cumulative mass loss rate normalized by the total mass loss rate (on the density colormap) and the cumulative kinetic luminosity normalized by the total kinetic luminosity (on the velocity colormap) to give a sense on where mass and kinetic energy are lost.
• Figure 4: Force multiplier maps of 6 selected simulations as a function of $z/x$ and $x$. The left column shows the simulations with the truncated disc SED while the right column shows the simulations with the full disc SED. From top to bottom the X-ray luminosity of the central source increases. The black solid line gives the sonic point at each radius.
• Figure 5: Top panels: Time series of density maps zoomed on the region where a transient ejection originates spanning the time before, during and after the transient ejection. Bottom panels: SEDs at the point circled on the top panels from which the transient ejection originated.