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We have measured the redshift evolution of the density of Lyman limit systems (LLS) in the intergalactic medium over the redshift range 0 20. 3 show the same redshift evolution as the LLS. When combined with previous results, our DLA sample is also consistent with a constant OmegaDLA= 9 x 10^ (-4) from z = 2 to z = 5. We have used the LLS number density evolution to compute the evolution in the mean free path of ionizing photons. We find a smooth evolution to z ~ 6, very similar in shape to that of Madau, Haardt & Rees (1999) but about a factor of two higher. Recent theoretical models roughly match to the z 6 in different ways, cautioning against extrapolating the fit to the mean free path outside the measured redshift range.
Songaila et al. (Fri,) studied this question.