Observed [N ii]/[C ii] flux ratios compared with model predictions. The green hatched range denotes the observed range for low-z galaxies. The horizontal dashed line shows our ALMA result on LESS J0332, where the emission-line fluxes are measured by integrating the best-fit Gaussian function. Dotted magenta lines denote the flux ratio at the red part (from  −350 km s^-1 to  −150 km s^-1) and blue part (from  −550 km s^-1 to  −350 km s^-1) of the lines. The red and blue lines show Cloudy model results as a function of Z_gas with log n_HII = 1.5 and 3.0 respectively, while solid and dashed lines denote the models with log U_HII =  −2.5 and  − 3.5, respectively.