Top: Fraction of H I, Mg⁺, Si⁺, Fe⁺, and C⁺ as a function of temperature, assuming collisional ionisation equilibrium (first column) and photoionisation equilibrium (other columns). Ionisation fractions have been computed with CLOUDY using solar abundance ratios and constant gas density. For photoionisation models, we use incident stellar radiation fields from BPASS 2.2.1 assuming solar metallicity and parametrised by various ionisation parameter values : log U = −4.5, −3.5, and −2.5 (see text for details). Bottom: Relative singly-ionised metal fractions with respect to hydrogen neutral fraction.