THE KECK plus MAGELLAN SURVEY FOR LYMAN LIMIT ABSORPTION. II. A CASE STUDY ON METALLICITY VARIATIONS

We present an absorption line analysis of the Lyman limit system (LLS) at z approximate to 3.55 in our Magellan/MIKE spectrum of PKS2000 - 330. Our analysis of the Lyman limit and full H I Lyman series constrains the total H I column density of the LLS (N(H I) = 10(18.0 +/- 0.25) cm(-2) for b(H I) >= 20 km s(-1)) and also the N(H I) values of the velocity subsystems comprising the absorber. We measure ionic column densities for metal-line transitions associated with the subsystems and use these values to constrain the ionization state (> 90% ionized) and relative abundances of the gas. We find an order of magnitude dispersion in the metallicities of the subsystems, marking the first detailed analysis of metallicity variations in an optically thick absorber. The results indicate that metals are not well mixed within the gas surrounding high z galaxies. Assuming a single-phase photoionization model, we also derive an N(H)-weighted metallicity, <[Si/H]> = - 1.66 +/- 0.25, which matches the mean metallicity in the neutral interstellar medium in high z damped Ly alpha systems (DLAs). Because the line density of LLSs is over 10x higher than the DLAs, we propose that the former dominate the metal mass-density at z similar to 3 and that these metals reside in the galaxy/intergalactic medium interface. Considerations of a multi-phase model do not qualitatively change these conclusions. Finally, we comment on an anomalously large O(0)/Si(+) ratio in the LLS that suggests an ionizing radiation field dominated by soft UV sources (e.g., a starburst galaxy). Additional abundance analysis is performed on the super-LLS systems at z approximate to 3.19.