The Physical Nature of the Lyman Limit Systems

We analyze Keck HIRES observations of a Lyman limit system at z=2.652 toward Q2231-00. These observations afford the most comprehensive study of the physical properties of a LL system to date. By comparing the ionic column densities for Fe^+, Fe^{++}, Si^+, and Si^{3+} against calculations derived from the CLOUDY software package, we have strictly constrained the ionization state of this system. This has enabled us to calculate accurate abundances of a Lyman limit system for the first time at z > 2, e.g., [Fe/H] = -0.5 \pm 0.1. We also derive a total hydrogen column density, log N(H) = 20.73 \pm 0.2, which is comparable to values observed for the damped lya systems. The system is special for exhibiting CII* 1335 absorption, allowing an estimate of the electron density, n_e = 6.5 \pm 1.3 x 10^{-2} cm^{-3}. Coupling this measurement with our knowledge of the ionization state, we derive the following physical properties: (1) hydrogen volume density, n_H = 5.9 \pm 1.2 x 10^{-2} cm^{-3}, (2) path length, l = 3 \pm 1.6 kpc, and (3) ionizing intensity, log J_{912} = -20.22 \pm 0.21. We point out that a number of the physical properties (e.g. [Fe/H], N(H), n_H) resemble those observed for the damped lya systems, which suggests this system may be the photoionized analog of a damped system. The techniques introduced in this Letter should be applicable to a number of Lyman limit systems and therefore enable a survey of their chemical abundances and other physical properties.