The Gunn-Peterson effect and the H I column density distribution of Lyman alpha forest clouds at z = 4

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Abstract
We present the results of a search for the Gunn–Peterson effect at z ~ 4. A model for the intensity distribution in spectral regions in the Lyman α forest avoiding strong absorption lines is described. This is used to constrain the possible contribution to the observed opacity from both a constant absorption component and a population of weak absorption lines (with neutral hydrogen column densities below a conservative completeness limit of NHI = 1013.75 cm–2). We find that if the neutral hydrogen column density distribution derived from the stronger absorption lines |$\,\text{taken} \,\text{as} \,\text{a} \,\text{power} \,\text{law}\, p(N_\text{HI}\propto N_\text{HI}^{-\beta}\,\text{with}\,\beta =1.7$| is extrapolated to values below NHI = 1013.75 cm–2, a significant Gunn–Peterson effect is required to produce an acceptable fit to the data. However, we find a good fit to the intensity distribution using a considerably flatter column density distribution below NHI = 1013.75 cm–2 (β = 1.3) extrapolated down to NHI = 1012 cm–2 with no Gunn–Peterson component. If higher quality spectra of 0000 – 263 reveal that the NHI power law with β = 1.7 does in fact extend down to NHI= 1013 cm–2, our results would then imply the detection of the long sought after Gunn–Peterson effect, with opacity τGP(z = 4) ≈0.04.

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