Nickolay Y. Gnedin, Nicholas Hollon
Cooling functions of cosmic gas are a crucial ingredient for any study of gas
dynamics and thermodynamics in the interstellar and intergalactic medium. As
such, they have been studied extensively in the past under the assumption of
collisional ionization equilibrium. However, for a wide range of applications,
the local radiation field introduces a non-negligible, often dominant,
modification to the cooling and heating functions. In the most general case,
these modifications cannot be described in simple terms, and would require a
detailed calculation with a large set of chemical species using a radiative
transfer code (the well-known code Cloudy, for example). We show, however, that
for a sufficiently general variation in the spectral shape and intensity of the
incident radiation field, the cooling and heating functions can be
\emph{approximated} as depending only on (1) the photo-dissociation rate of
molecular hydrogen, (2) the hydrogen photo-ionization rate, and (3) the
photo-ionization rate of OVIII; more complex and more accurate approximations
also exist. Such dependence is easy to tabulate and implement in cosmological
or galactic-scale simulations, thus economically accounting for an important
but rarely-included factor in the evolution of cosmic gas. We also show a few
examples where the radiation environment has a large effect, the most
spectacular of which is a quasar that suppresses gas cooling in its host halo
without any mechanical or non-radiative thermal feedback.
View original:
http://arxiv.org/abs/1201.5116
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