THE KECK plus 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-H I and associated metal-line transitions for 157 LLSs at z(LLS) = 1.76-4.39 restricted to 10(17.3) cm(-2) <= N-H I < 10(20.3) cm(-2). An empirical analysis of ionic ratios indicates an increasing ionization state of the gas with decreasing N-H I and that the majority of LLSs are highly ionized, confirming previous expectations. The Si+/H-0 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 similar to 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 (approximate to 5%-10%) that may have super-solar abundances. High Si+/Fe+ ratios suggest an alpha-enhanced medium, whereas the Si+/C+ ratios do not exhibit the super-solar enhancement inferred previously for the Ly alpha forest.