The HST/ACS+WFC3 Survey for Lyman Limit Systems II: Science
  We present the first science results from our Hubble Space Telescope Survey
for Lyman limit absorption systems (LLS) using the low dispersion spectroscopic
modes of the Advanced Camera for Surveys and the Wide Field Camera 3. Through
an analysis of 71 quasars, we determine the incidence frequency of LLS per unit
redshift and per unit path length, l(z) and l(x) respectively, over the
redshift range 1 < z< 2.6, and find a weighted mean of l(x)=0.29 +/-0.05 for
2.0 < z < 2.5 through a joint analysis of our sample and that of Ribaudo et al.
(2011). Through stacked spectrum analysis, we determine a median (mean) value
of the mean free path to ionizing radiation at z=2.4 of lambda_mfp =
243(252)h^(-1) Mpc, with an error on the mean value of +/- 43h^(-1) Mpc. We
also re-evaluate the estimates of lambda_mfp from Prochaska et al. (2009) and
place constraints on the evolution of lambda_mfp with redshift, including an
estimate of the "breakthrough" redshift of z = 1.6. Consistent with results at
higher z, we find that a significant fraction of the opacity for absorption of
ionizing photons comes from systems with N_HI <= 10^{17.5} cm^(-2) with a value
for the total Lyman opacity of tau_lyman = 0.40 +/- 0.15. Finally, we determine
that at minimum, a 5-parameter (4 power-law) model is needed to describe the
column density distribution function f(N_HI, X) at z \sim 2.4, find that
f(N_HI,X) undergoes no significant change in shape between z \sim 2.4 and z
\sim 3.7, and provide our best fit model for f(N_HI,X).
