Predicted ionic column densities as a function of ionisation parameter U,calculated for a location 200 kpc from the Sun in the direction of PKS2155-304. Black lines and points indicate hydrogen; blue lines and points indicate carbon, red indicate oxygen, and purple indicate silicon. Circles indicate C ii and Si ii, triangles for Si iii, squares indicate C iv and O iv, and stars indicate O v. The observed values are indicated by the larger symbols near log U = −3.0, at which value both C iv and Si iv match the observations. The thick parts of the lines show where the prediction matches the range of observed values.