Line flux ratio between HNC and HCN as a function of the infrared luminosity. The red star is the mean ratio found in our sources and the gray diamond is the mean ratio found by Spilker et al. (2014) using a stacking of the cycle-0 SPT SMG spectra. The black square is the upper limit determined for SPT0551-50. For the other sources, the individual ratio cannot be constrained, since both lines are too weak to derive an upper or a lower limit. The blue filled circles represent the local sample of (Costagliola et al. 2011, 1-0 transition). The green triangles are the 3-2 transitions in the Arp220, Mrk231, and NGC4418 starbursts (Aalto et al. 2007). The purple crosses are the two z ∼ 1.5 lensed star-forming galaxies of Oteo et al. (2016) and the yellow plus is the APM08279+5255 quasar (Guélin et al. 2007, see also Riechers et al. 2010 for the 6-5 transition). The orange downwards triangles are two H-ATLAS sources provided in Yang’s Thesis (priv. comm.).