THE GUNN-PETERSON EFFECT AND THE H-I COLUMN DENSITY DISTRIBUTION OF LYMAN ALPHA-FOREST CLOUDS AT Z=4

We present the results of a search for the Gunn-Peterson effect at z approximately 4. A model for the intensity distribution in spectral regions in the Lyman alpha-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 N(H1) = 10(13.75) cm-2). We find that if the neutral hydrogen column density distribution derived from the stronger absorption lines [taken as a power law p(NH1) is-proportional-to N(H1)-beta with beta = 1.7] is extrapolated to values below N(H1) = 10(13.75) 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 N(H1) = 10(13.75) cm-2 (beta = 1.3) extrapolated down to N(H1) = 10(12) cm-2 with no Gunn-Peterson component. If higher quality spectra of 0000-263 reveal that the N(H1) power law with beta = 1.7 does in fact extend down to N(H1) = 10(13) cm-2, our results would then imply the detection of the long sought after Gunn-Peterson effect, with opacity tau(GP) (z = 4) almost-equal-to 0.04.