The keck + magellan survey for lyman limit absorption. III. sample definition and column density measurements

Abstract

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 NH I and associated metal-line transitions for 157 LLSs at z 1.76 4.39 zLLS = restricted to 1017.3cm-2 ≤ NH1 <1020.3 cm-2 I An empirical analysis of ionic ratios indicates an increasing ionization state of the gas with decreasing NH 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 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%¡V10%) that may have super-solar abundances. High Si+/Fe+ ratios suggest an enhanced medium, whereas the Si+/C+ ratios do not exhibit the super-solar enhancement inferred previously for the Lyα forest.