Top panel: 12.7/11.3 μm PAH EW ratio vs the IRAS 12 μm/IRS 12 μm flux ratios for the HERUS sample. Green, blue, and red symbols correspond to group 1, 2, and 3 of sources (see Sect. 4.1. Bottom panel: same as the top panel but using the GOALS sample. The grey solid and dashed horizontal lines correspond to the average and 3σ values found by Hernán-Caballero et al. (2020) for star-forming galaxies. The blue and red solid horizontal lines represent the average values found for groups 2 and 3 of ULIRGs (see Sect. 4.1).