Locally optimally emitting clouds and the origin of quasar emission lines

Jack Baldwin, Gary Ferland, Kirk Korista, Dima Verner

Research output: Contribution to journalArticlepeer-review

289 Scopus citations


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.

Original languageEnglish
Pages (from-to)L119-L122
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 20 1995

Bibliographical note

Funding Information:
We are grateful to Mark Bottorff, Moshe Elitzur, and the referee Simon Morris for helpful comments. This work was supported by the NSF (AST 93-19034), NASA (NAGW-3315), and STScI grant GO-2306.


  • Quasars: emission lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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