The Physical Nature of the Lyman-Limit Systems

Abstract

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 Lyman-limit 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+/-0.1. We also derive a total hydrogen column density of logN =20.73+/-0.2, which is comparable to values observed for the damped Lyalpha systems. The system is special for exhibiting C II* lambda1335 absorption, allowing us to estimate the electron density, n_e=6.5+/-1.3x10^-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+/-1.2x10^-2 cm^-3, (2) path length l=3+/-1.6 kpc, and (3) ionizing intensity logJ_912=-20.22+/-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 Lyalpha systems, which suggests that 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.