Ionization fractions of different Si ionization states for different gas densities and temperatures, as calculated from a Cloudy ionization model (including photoionization and collisional ionization) of circumgalactic gas at z = 0 using the local UV background. The upper panel shows a model for log T = 4.2, the lower panel a model for log T = 4.4. The Si iii ion represents the dominant ionization state in the density range that is characteristic for circumgalactic gas structures that are embedded in hot coronal gas (log n_H = −2.0 to −3.5).