What AGN revergeration maps tell us: plasma simulations of dense accreting gas

Reading BLR reverberation maps - next generation plasma simulations. Reverberation measurements of black hole masses have proven to be an essential means of probing galaxy and black hole formation across cosmic time. Brad Peterson's Cycle 21 large program, Mapping the AGN Broad-line region by reverberation, will create the best-ever set of line - continuum measurements documenting how the emitting gas responds to changes in the ionizing radiation field. This project will "clarify the nature of the broad-line region, its role in the apparently complicated accretion/outflow process, and determine definitively the veracity and accuracy of the AGN reverberation-based black hole masses that are now a key component of any study of black hole growth, evolution, and the role of feedback in the evolution of galaxies." I am not involved in this program, but Cloudy, the spectral simulation code I have been developing, is uniquely suited to help interpret these observations. Emission from BLR gas is difficult to predict due to the gas's high density, large column density, and its exposure to an intense non-thermal ionizing continuum. The gas producing the spectrum is far from equilibrium, and its physical state depends on a host of microphysical processes. These conditions mean that the gas is more similar to a Tokamak plasma than the interstellar medium. The large column density means that most lines are optically thick, therefore line transfer processes are important. The non-thermal SED introduces a number of exotic high-energy processes. Although these are all complications, advances in computer power, the atomic/molecular database, and radiative transfer methods, render the problem solvable. The graduate texts Netzer (2013), Osterbrock & Ferland (2006, hereafter AGN3), and Peterson (1997), review this field. Cloudy, available at www.nublado.org, is an open-source plasma simulation code that simulates conditions in a non-equilibrium gas and predicts its spectrum. The code incorporates physical processes from first principles. The goal is to simulate the ionization, level populations, chemistry, and thermal state, over all extremes of density and temperature. Our approach, working from fundamental processes, means that Cloudy can be applied to such diverse regions as the corona of a star, the intergalactic medium, or the accretion disk near the supermassive black hole in a luminous quasar. As a result, the code is widely used, with nearly 200 papers citing its documentation each year. Ferland et al. (2013, hereafter F13, published in an Open Access journal) summarizes the current status of Cloudy, and its 2013 release. This theory proposal requests support for development of Cloudy along two independent lines in direct support of BLR observations. The first is to include the effects of electron scattering on the formation of all lines. This has been neglected or underestimated in the past. Estimates show that it is quite important and will affect both the intensity and profile of optical/UV H I lines. The second is the incorporation of a large magnetic-fusion atomic database into the treatment of recombination. This will affect the intensities of all heavy-element lines in the optical/UV spectrum. I am submitting this as a two-year theory project, with a "small" budget in each year, in recognition of these two different problems. They are independent.