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. Through stacked spectrum analysis, we determine a median (mean) value of the mean free path to ionizing radiation at z = 2.4 of λmfp912 = 243(252) h72 Mpc -1, with an error on the mean value of ±43 h72 -1Mpc. We also re-evaluate the estimates of λ mfp912 from Prochaska et al. and place constraints on the evolution of λmfp912 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 ≤1017.5 cm-2 with a value for the total Lyman opacity of τeffLyman = 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 ∼ 2.4, find that f(N H I , X) undergoes no significant change in shape between z ∼ 2.4 and z ∼ 3.7, and provide our best fit model for f(N HI , X).