Helium (Reionisation)

From Eric's Wiki

If the ionising spectra is uniform, HeII and HI Lyα ratios are constant. But large fluctuations are observed out to z=4, after which there is an increase in the HeII opacity. A model for varying the local spectrum is fairly uniform SF (which ionises HI) with rare QSO's (which ionise the helium) (BHVC05).

Reimers05 report HeII absorption in a QSO spectrum from z=2.8 systems. The HeII is not easily modelled by the HI optical depth in the voids. They infer that the HeII reionisation is thus not complete (still some HeII to turn into HeIII).

Fluctuations in the ratio of HeII to HI is an indicator of the hardness of the local radiation field. Absorption of a radiation field by HI leads to hardening of the spectrum which could explain the observed fluctuations. However, TM06 find from their simulations that the hardening of the spectrum does occur, but it is not enough to explain the abundance or range of fluctuations.

--Etittley 16:11, 24 June 2007 (BST)

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