The escape of Lyman-alpha radiation from a multiphase interstellar medium

The Eddington approximation is used to model the transfer of radiation within a multiphase scattering medium. An estimate is thereby derived for the fraction of H Ly-alpha photons that escape from a galaxy with a two-phase interstellar medium in which dusty gas clouds lie embedded within an intercloud medium of negligible absorption and scattering coefficients. Under suitable conditions, Ly-alpha photons may actually suffer less attenuation than radiation which is not resonantly scattered, due to the fact that such photons spend most of their time in the intercloud medium as they bounce their way from one interstellar cloud surface to the next. This effect may lead to an enhancement in the observed line-to-continuum ratio, and to a Ly-alpha line profile which may be considerably narrower than that which would emerge from uniform medium of the same total atomic column density.