Effects of external radiation fields on line emission - Application to star-forming regions

Marios Chatzikos, G. J. Ferland, R. J.R. Williams, Ryan Porter, P. A.M. Van Hoof

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


A variety of astronomical environments contain clouds irradiated by a combination of isotropic and beamed radiation fields. For example, molecular clouds may be irradiated by the isotropic cosmic microwave background, as well as by a nearby active galactic nucleus. These radiation fields excite atoms and molecules and produce emission in different ways. We revisit the escape probability theorem and derive a novel expression that accounts for the presence of external radiation fields. We show that when the field is isotropic the escape probability is reduced relative to that in the absence of external radiation. This is in agreement with previous results obtained under ad hoc assumptions or with the two-level system, but can be applied to complex many-level models of atoms or molecules. This treatment is in the development version of the spectral synthesis code CLOUDY. We examine the spectrum of a Spitzer cloud embedded in the local interstellar radiation field and show that about 60% of its emission lines are sensitive to background subtraction. We argue that this geometric approach could provide an additional tool toward understanding the complex radiation fields of starburst galaxies.

Original languageEnglish
Article number122
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 20 2013


  • ISM: clouds
  • atomic processes
  • line: formation
  • methods: numerical
  • radiative transfer

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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