Observed [Nii]/[Cii] flux ratio compared with model predictions for different density n and ionisation parameter U (see Nagao et al. 2012). The green hatched range denotes the observed range for low-redshift galaxies. The pink hatched range shows the observed ALMA ratio for ALESS 73.1 with its uncertainty. The red and blue lines show Cloudy model results as a function of Z_gas with log n_{H [ II ]} = 1.5 and 3.0 respectively, while solid and dashed lines denote the models with log U_{H [ II ]} −2.5 and −3.5, respectively. We conclude that the metallicity in this gas rich disk is likely to be close to solar.