Line ratios observed towards nearby galaxies and the Orion B molecular cloud. The upper and lower panels include line ratios which are proposed dense gas and UV-illumination tracers, respectively. A point is plotted for individual measures, while a range of values is given for the Ultra-Luminous Infra-Red Galaxies (ULIRG), Active Galactic Nuclei (AGN), and starburst samples, which contain measures for several galaxies. The ULIRGs include Arp 220 and Mrk 231, while AGNs include M 51, NGC 1068 and NGC 7469, and the starburst galaxies include M 83, NGC 253 and M 82. The sources are sorted by decreasing spatial resolution, which is given at the bottom of the figure for each source. The Orion B values correspond to the ratios of the lines integrated over the full region to simulate a 10 pc spatial resolution allowing a better comparison with the other galaxies.