Fractal Quasar Clouds

Abstract

This paper examines whether a fractal cloud geometry can reproduce the emission-line spectra of active galactic nuclei (AGNs). The nature ofthe emitting clouds is unknown, but many current models invoke varioustypes of magnetohydrodynamic confinement. Recent studies have arguedthat a fractal distribution of clouds, in which subsets of clouds occurin self-similar hierarchies, is a consequence of such confinement.Whatever the confinement mechanism, fractal cloud geometries are foundin nature and may be present in AGNs too. We first outline how a fractalgeometry can apply at the center of a luminous quasar. Scaling laws arederived that establish the number of hierarchies, typical sizes, columndensities, and densities. Photoionization simulations are used topredict the integrated spectrum from the ensemble. Direct comparisonwith observations establishes all model parameters so that the finalpredictions are fully constrained. Theory suggests that denser cloudsmight form in regions of higher turbulence and that larger turbulenceresults in a wider dispersion of physical gas densities. An increase inturbulence is expected deeper within the gravitational potential of theblack hole, resulting in a density gradient. We mimic this densitygradient by employing two sets of clouds with identical fractalstructuring but different densities. The low-density clouds have a lowercolumn density and large covering factor similar to the warm absorber.The high-density clouds have high column density and smaller coveringfactor similar to the broad-line region (BLR). A fractal geometry cansimultaneously reproduce the covering factor, density, column density,BLR emission-line strengths, and BLR line ratios as inferred fromobservation. Absorption properties of the model are consistent with theintegrated line-of-sight column density as determined from observationsof X-ray absorption, and when scaled to a Seyfert galaxy, the model isconsistent with the number of multiple UV absorption components observedin them. Rough estimates show that about one in 100 of the galaxies thatharbor a supermassive black hole will show activity, assuming thatmaterial needs to be within its EUV continuum emitting radius foractivity to occur. This is close to the observationally determined dutycycle. Stochastic feeding of the central engine of fractal clouddistribution of material may therefore account for continuum variationsand long-term activity. The total cloud mass is much larger than thatmeasured in ionized gas alone since the clouds are mutuallyself-shielding.