The physical nature of the Lyman-limit systems

We analyze Reck HIRES observations of a Lyman-limit system at z = 2.652 tow ard Q2231-00. These observations afford the most comprehensive study of the physical properties of a Lyman-limit system to date. By comparing the ioni c column densities for Fe+, Fe++, Si+, and Si3+ against calculations derive d from the CLOUDY software package, we have strictly constrained the ioniza tion state of this system. This has enabled us to calculate accurate abunda nces of a Lyman-limit system for the first time at z > 2, e.g., [Fe/H] = -0 .5 +/- 0.1. We also derive a total hydrogen column density of log N (H) = 2 0.73 +/- 0.2, which is comparable to values observed for the damped Ly alph a systems. The system is special for exhibiting C II* lambda 1335 absorptio n, allowing us to estimate the electron density, n(e) = 6.5 +/- 1.3 x 10(-2 ) cm(-3). Coupling this measurement with our knowledge of the ionization st ate, we derive the following physical properties: (1) hydrogen volume densi ty n(H) = 5.9 +/- 1.2 x 10(-2) cm(-3), (2) path length l = 3 +/- 1.6 kpc, a nd (3) ionizing intensity log J(912) = -20.22 +/- 0.21. We point out that a number of the physical properties (e.g., [Fe/H], N(H), n(H)) resemble thos e observed for the damped Ly alpha systems, which suggests that this system may be the photoionized analog of a damped system. The techniques introduc ed in this Letter should be applicable to a number of Lyman-limit systems a nd therefore enable a survey of their chemical abundances and other physica l properties.