Locally Optimally Emitting Clouds and the Origin of Quasar Emission Lines

Citations Over TimeTop 10% of 1995 papers

Abstract

The similarity of quasar line spectra has been taken as an indication that the emission line clouds have preferred parameters, suggesting that the environment is subject to a fine-tuning process. We show here that the observed spectrum is a natural consequence of powerful selection effects. We computed a large grid of photoionization models covering the widest possible range of cloud gas density and distance from the central continuum source. For each line only a narrow range of density and distance from the continuum source results in maximum reprocessing efficiency, corresponding to "locally optimally emitting clouds" (LOCs). These parameters depend on the ionization and excitation potentials of the line and its thermalization density. The mean QSO line spectrum can be reproduced by simply adding together the full family of clouds, with an appropriate covering fraction distribution. The observed quasar spectrum is a natural consequence of the ability of various clouds to reprocess the underlying continuum, and can arise in a chaotic environment with no preferred pressure, gas density, or ionization parameter.

Related Papers

• → Ionized Absorbers in Active Galactic Nuclei: The Role of Collisional Ionization and Time‐evolving Photoionization(1999)104 cited
• → The emission-line regions in broad absorption line quasars(1986)49 cited
• → Photoionization - interaction of atoms with strong fields(1988)4 cited
• → Variability of emission lines in quasar spectra(1986)2 cited
• → The ionization equilibrium of iron in H II regions(2003)