SIMBAD references

We present an absorption-line survey of optically thick gas clouds–Lyman Limit Systems (LLSs)–observed at high dispersion with spectrometers on the Keck and Magellan telescopes. We measure column densities of neutral hydrogen N_H1 and associated metal-line transitions for 157 LLSs at Z_LLS = 1.76 - 4.39 restricted to 10⁷³r/cm2 ≤ N_HI < 10^20.3. An empirical analysis of ionic ratios indicates an increasing ionization state of the gas with decreasing N_H1 and that the majority of LLSs are highly ionized, confirming previous expectations. The Si⁺/H⁰ ratio spans nearly four orders of magnitude, implying a large dispersion in the gas metallicity. Fewer than 5% of these LLSs have no positive detection of a metal transition; by z∼3 nearly all gas that is dense enough to exhibit a very high Lyman limit opacity has previously been polluted by heavy elements. We add new measurements to the small subset of LLS (≈5%-10%) that may have super-solar abundances. High Si⁺/Fe⁺ ratios suggest an α-enhanced medium, whereas the Si⁺/C⁺ ratios do not exhibit the super-solar enhancement inferred previously for the Lyα forest.