Damped Lyman alpha systems and galaxy formation models - II. High ions and Lyman-limit systems

We investigate a model for the high-ionization state gas associated with observed damped Lyman alpha systems, based on a semi-analytic model of galaxy formation set within the paradigm of hierarchical structure formation. In our model, the hot gas in haloes and subhaloes is assumed to be in a multiphase medium which gives rise to C IV absorption, while the low-ionization state gas is associated with the cold gas in galaxies. The model matches the distribution of C IV column densities if we assume that the hot gas has a mean metallicity log C/H=-1.5, which is the observed mean metallicity of damped systems. The same model then leads naturally to kinematic properties that are in good agreement with the data, for both the low- and high-ionization state gas. We examine the contribution of both hot and cold gas to subdamped systems (N-H I>4x10(19) cm(-2)) and suggest that the properties of these systems can be used as an important test of the model. We expect that sub-DLA systems will generally be composed of a single gas disc and thus predict that they should have markedly different kinematics from the damped systems. We also find that the frequency of absorbers drops dramatically for column densities below 4x10(19) cm(-2). These results are a consequence of our model for damped Lyman alpha systems and we believe they are a generic prediction of multicomponent models. Finally, we find that hot halo gas produces less than one-third of Lyman-limit systems at a redshift of 3. We model the contribution of mini-haloes (haloes with virial velocities less than or equal to35 km s(-1)) to Lyman-limit systems and find that they may contain as much gas as is observed in these systems. However, if we adopt realistic models of the gas density distribution we find that these systems are not a significant source of Lyman-limit absorption. Instead we suggest that uncollapsed gas outside of virialized haloes is responsible for most of the Lyman-limit systems at high redshift.