The Keck+Magellan Survey for Lyman Limit Absorption III: Sample Definition and Column Density Measurements

TL;DR

This survey of 157 Lyman Limit Systems at high redshift measures hydrogen and metal-line column densities, revealing high ionization, metallicity dispersion, and metal enrichment over cosmic time, with implications for galaxy evolution.

Contribution

It provides new high-resolution measurements of NHI and metal lines in LLSs, expanding understanding of their ionization states and metallicities at z=1.76-4.39.

Findings

Most LLSs are highly ionized.

Large metallicity dispersion among LLSs.

Heavy element pollution increases with redshift.

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 NHI and associated metal-line transitions for 157 LLSs at z=1.76-4.39 restricted to 10^17.3 < NHI < 10^20.3. An empirical analysis of ionic ratios indicates an increasing ionization state of the gas with decreasing NHI 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~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 alpha-enhanced medium whereas the Si^+/C^+ ratios do not exhibit the super-solar enhancement inferred previously for the Lya forest.