[O III]_88 μm/[N II]_122 μm luminosity ratio as a function of metallicity Z and hardness of the photoionizing radiation, parameterized as the temperature of an equivalent black body, T_*, computed from Cloudy models assuming a gas density of n = 180 cm⁻³. The shaded area marks the observed line ratio and its 1-σ confidence interval. The line ratio is nearly independent of metallicity at Z ≲ 0.5 Z_⊙ (see Eq. (17)), while it is strongly sensitive to the hardness of the ionizing radiation. Our observations point to a value of T_* = 21 000–32 000 K.