Correlation between L_IR and L_{H2O(211 − 202)} for local (black symbols), high-redshift star-forming SMGs (blue symbols), and QSO/AGN (red). z-GAL sources are marked in green. The fitting of the correlation is represented by the solid orange line. The posterior distribution of the slope is shown in the inset. The plot is adapted from Yang et al. (2016, see references therein). The new fit is performed by including the z-GAL sources (green symbols) as well as the sources from the literature.