SIMBAD references

We perform a semi-automated survey for τ₉₁₂≥ 2 Lyman limit systems (LLSs) in quasar spectra from the Sloan Digital Sky Survey, Data Release 7. From a starting sample of 2473 quasars with z_em= 3.6-5.0, we analyze 429 spectra meeting strict selection criteria for a total redshift path Δz = 93.8 and identify 190 intervening systems at z_LLS≥ 3.3. The incidence of τ₉₁₂≥ 2 LLSs per unit redshift, ℓ_τ≥2(z), is well described by a single power law at these redshifts: ℓ_τ≥2(z)= C_LLS [(1+z)/(1+z_*)]γ_LLS, with z_*≡ 3.7, C_LLS= 1.9±0.2, and γ_LLS= 5.1±1.5 (68% c.l.). These values are systematically lower than previous estimates (especially at z < 4) but are consistent with recent measurements of the mean free path to ionizing radiation. Extrapolations of this power law to z = 0 are inconsistent with previous estimations of ℓ(z) at z < 1 and may indicate a break at z ≈ 2, similar to that observed for the Lyα forest. Our results also indicate that the systems giving rise to LLS absorption decrease by ≈ 50% in comoving number density and/or physical size from z = 4 to 3.3, perhaps due to an enhanced extragalactic ultraviolet background. The observations place an integral constraint on the H I frequency distribution f(N_ H I_, X) and indicate that the power-law slope β≡dlnf(N_H i,X)/dlnN_H i is likely shallower than β = -1 at {N_H i~10^{18} cm^-2}. Including other constraints on f(N_ H I_, X) from the literature, we infer that β is steeper than β = -1.7 at {N_H i~10^{15} cm^-2}, implying at least two inflections in f(N_ H I_, X). We also perform a survey for proximate LLSs (PLLSs) and find that ℓ_PLLS(z) is systematically lower (≈ 25%) than intervening systems. Finally, we estimate that systematic effects impose an uncertainty of 10%-20% in the ℓ(z) measurements; these effects may limit the precision of all future surveys.