THE KECK + MAGELLAN SURVEY FOR LYMAN LIMIT ABSORPTION. III. SAMPLE DEFINITION AND COLUMN DENSITY MEASUREMENTS

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 ${N}_{{\rm{H}}\;{\rm{I}}}$ and associated metal-line transitions for 157 LLSs at ${z}_{\mathrm{LLS}}=1.76-4.39$ restricted to ${10}^{17.3}\;{\mathrm{cm}}^{-2}\leqslant {N}_{{\rm{H}}\;{\rm{I}}}\lt {10}^{20.3}\;{\mathrm{cm}}^{-2}.$ An empirical analysis of ionic ratios indicates an increasing ionization state of the gas with decreasing ${N}_{{\rm{H}}\;{\rm{I}}}$ 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 $z\sim 3,$ 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.