Effects of External Radiation Fields on Line Emission - Application to
  Star-forming Regions
  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 (CMB), as
well as by a nearby active galactic nucleus (AGN). 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 percent 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.
