Predicted C III] λ1909 rest-frame equivalent width as a function of [O III] λλ4959,5007/[O II] λ3727 for a gas-phase metallicity 12 + log(O/H) = 7.65 (~1/10 solar), electron density N_e = 1000 cm⁻³, varying ionization parameters log U = −3, −2.5, −2, −1.5, −1 (dashed colored lines from left to right) and escape fractions from zero (top) approaching unity (bottom). The models assume a BPASS SED for a population of 1 Myr, as in Fig. 2. The observations of J1154+2443 are well reproduced with a fairly high ionization parameter and with the observed escape fraction f_esc = 46%.