Contour plots of the ratio between the [CII] line and the cumulative flux of CIII]1909 Å (upper panels), [OIII]5007 Å (middle panels), and Hα (lower panels) as a function of the ionisation parameter (U) and the H number density (n). The gas metallicity is fixed to Z = 0.55 Z_⊙. Left panels: CLOUDY models using BPASS with a continuous SFH. Right panels: CLOUDY models using SB99 with a quenched SFH (Δt_q = 5 Myr).