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Paper: Numerical Simulations of Plasmas and Their Spectra
Volume: 6, Twenty Years of ADASS
Page: 33
Authors: Ferland, G. J.; Korista, K. T.; Verner, D. A.
Abstract: This review centers on the development and application of Cloudy, a large-scale code designed to compute the spectrum of gas in photoionization or collisional balance. Such plasma is far from equilibrium, and its conditions are set by the balance of a host of microphysical processes. The development of Cloudy is a three-pronged effort requiring advances in the underlying atomic data base, the numerical and computational methods used in the simulation, culminating in the application to astronomical problems. These three steps are strongly interwoven. A complete simulation involves many hundreds of stages of ionization, many thousands of levels, with populations determined by a vast sea of atomic/molecular processes, many with accurate cross sections and rate coefficients only now becoming available. The scope of the calculations and the numerical techniques they use, can be improved as computers grow ever faster, since previous calculations were naturally limited by the available hardware. The final part is the application to observations, driven in part by the revolution in quality of spectra made possible by advances in instrumentation. The galactic nebulae represent laboratories for checking whether the physics of nebulae is complete and for testing galactic chemical evolution theories, and can validate the analysis methods to be used on the quasars. Models of ablating molecular clouds, the likely origin of some gas in quasars, are tested by studies of the Orion complex. Finally, the quasars themselves present the ultimate challenge: to deduce the composition of their emitting gas from the spectrum, determine the dependencies of this spectrum on the shape of the ionizing continuum, correlating this with observed changes in the spectral energy distribution, and finally to understand its dependencies on luminosity (the Baldwin effect). Once the emission line regions of quasars are understood, we will have a direct probe of the z <= 5 universe. Cloudy is widely used in the astronomical community, with roughly 50 papers per year employing it. The code is a general spectroscopic tool whose development has an impact well beyond out specific studies.
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