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 halos and sub-halos is assumed to be in a multi-phase medium which gives rise to CIV absorption , while the low-ionization state gas is associated with the cold gas in galaxies .
The model matches the distribution of CIV 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 sub-damped systems ( N _ { HI } > 4 \times 10 ^ { 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 disk and thus predict that they should have markedly different kinematics than the damped systems .
We also find that the frequency of absorbers drops dramatically for column densities below 4 \times 10 ^ { 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 multi-component models .

Finally , we find that hot halo gas produces less than one third of Lyman limit systems at redshift three .
We model the contribution of mini-halos ( halos with virial velocities \leq 35 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 halos is responsible for most of the Lyman limit systems at high redshift .