The Evolution of Lyman Limit Absorption Systems to Redshift Six
  We have measured the redshift evolution of the density of Lyman limit systems
(LLS) in the intergalactic medium over the redshift range 0 < z < 6. We have
used two new quasar samples to (1) improve coverage at z ~ 1, with GALEX grism
spectrograph observations of 50 quasars with 0.8 < z_em < 1.3, and (2) extend
coverage to z ~ 6, with Keck ESI spectra of 25 quasars with 4.17 < z_em < 5.99.
Using these samples together with published data, we find that the number
density of LLS per unit redshift, n(z), can be well fit by a simple evolution
of the form n(z) = n_3.5 [(1+z)/4.5]^gamma, with n_3.5 = 2.80 +/- 0.33 and
gamma = 1.94^(+0.36)_(-0.32) for the entire range 0 < z < 6. We have also
reanalyzed the evolution of damped Lyman alpha systems (DLAs) in the redshift
range 4 < z < 5 using our high-redshift quasar sample. We find a total of 17
DLAs and sub-DLAs, which we have analyzed in combination with published data.
The DLAs with log (HI column density) > 20.3 show the same redshift evolution
as the LLS. When combined with previous results, our DLA sample is also
consistent with a constant Omega_DLA= 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 data but diverge from the
measured power law at z > 6 in different ways, cautioning against extrapolating
the fit to the mean free path outside the measured redshift range.
