Implications of infalling Fe II - emitting clouds in active galactic nuclei: anisotropic properties

TL;DR

The paper proposes that Fe II emission in AGN originates from infalling, low-ionization clouds whose shielded faces dominate the observed spectrum. Using Cloudy to model shielded-face emission, it shows that the force multiplier on the illuminated face is extremely large ($σ/σ_T \\sim 10^3$–$10^4$), so radiative pressure would drive outflows unless the clouds have substantial shielding and high column densities, making infall viable. A key result is a minimum infall column density that scales with the Eddington ratio, linking $L/L_{Edd}$ to Fe II/Hβ and to Eigenvector 1; UV Fe II is more beamed inward than optical Fe II, helping reconcile observed line ratios. The work predicts a near-constant Fe II/Hβ saturation around ~3 at high $N_H$ or $L/L_{Edd}$, requires a large covering factor, and suggests the infalling Fe II–emitting material contributes meaningfully to the AGN mass budget, with implications for BLR structure and the anisotropy of Fe II emission.

Abstract

We investigate consequences of the discovery that Fe II emission in quasars, one of the spectroscopic signatures of "Eigenvector 1", may originate in infalling clouds. Eigenvector 1 correlates with the Eddington ratio L/L_Edd so that Fe II/Hbeta increases as L/L_Edd increases. We show that the "force multiplier", the ratio of gas opacity to electron scattering opacity, is ~ 10^3 - 10^4 in Fe II-emitting gas. Such gas would be accelerated away from the central object if the radiation force is able to act on the entire cloud. As had previously been deduced, infall requires that the clouds have large column densities so that a substantial amount of shielded gas is present. The critical column density required for infall to occur depends on L/L_Edd, establishing a link between Eigenvector 1 and the Fe II/Hbeta ratio. We see predominantly the shielded face of the infalling clouds rather than the symmetric distribution of emitters that has been assumed. The Fe II spectrum emitted by the shielded face is in good agreement with observations thus solving several long-standing mysteries in quasar emission lines.

Figures (5)

• Figure 2: Force multiplier, evaluated at the illuminated face of the cloud, over the photon flux-density plane. The shaded ellipse is the region capable of reproducing Fe ii observed emission 2004ApJ...615..610B.
• Figure 3: Gas kinetic temperature, the computed force for the radiative acceleration, and the force for optically thin electron scattering, are shown as a function of column density from the illuminated face. Ionizing radiation enters from the left. The two lines showing the integrated force intercept at a column density of $N(\mathrm{H}) \approx 1.2\times 10^{24} \hbox{${\rm cm}^{-2}\,$}$, the point where the cloud would have neutral buoyancy for an AGN at the Eddington Limit.
• Figure 4: Predicted total (solid line) and outward (dashed line) emission relative to the total H$\beta$ is shown as a function of column density. H$\beta$ is isotropically emitted for small column densities while the optical Fe ii band remains nearly isotropic for all column densities. The UV Fe ii band is predominantly inwardly beamed due to the larger optical depth in these lines.
• Figure 5: Upper panel compares the total (solid) and outward (dotted) Fe ii emission of our standard cloud. The lower panel shows the ratio of the outward to total emission.
• Figure 6: Red smoother line is the outward Fe ii emission from the standard model broadened to a FWHM of $900 \hbox{${\rm km}{\rm s^{-1}}\,$}$ while the black spiky line is the observed Fe ii template. Intermediate-line region clouds account for the optical emission while allowing part of the UV emission to come from the inner broad-line region.