Glossary (Reionisation)
From Eric's Wiki
Confusing terminology: metagalactic flux (flux outside galaxies). But what really is flux? In an isotropic energy field, the mean flux F = 0. J is the mean intensity over all directions, where intensity I is the energy coming from a given direction, per unit steradian, per unit frequency, per unit area. When people talk of metagalactic flux, they really mean metagalactic intensity.
Confusing terminology: what do we call the gas not contained in galaxies? The intergalactic medium (IGM) implies gas between the galaxies, which is probably a good enough term. But you might also see intracluster medium or intercluster medium (both ICM!) which distinguishes the IGM from gas within a cluster of galaxies to gas between clusters. At high enough redshifts, though, there won't be many clusters, or even galaxies as we think of them today. So IGM is sufficient.
Softness Parameter: the ratio of the photoionisation rates of HI to HeII.
Lymann Continuum ?
On the spot approximation allows for ignoring the contribution recombination to a radiation field. Since both the recombination spectrum and absorption cross-sections are so strongly peaked just beyond the Lymann edge, photons released by recombination are absorbed by even a very small amount of HI. Hence, the photons tend not to go far. Recombination produces a photon and HI. The photon is lost nearby when it destroys the HI. This leads to no net change in the number density of photons and neutral particles. Need some math here. Since the photon is not absorbed instantaneously, Jν will be softened, which should change Γ and, ultimately, fHI.
--Etittley 11:56, 25 June 2007 (BST)
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