Diagnostic diagram of PAH band ratios 11.3/7.7 vs. 6.2/7.7 μm for the 3″ × 3″ (∼500 pc) apertures as illustrated in Figure 1. The red rectangles, blue star, and slate-gray hexagons represent the measurements of apertures in ESO137-G034, MCG-05-23-016, and NGC 3081, respectively, where points with a black dot pertain to the innermost aperture (i.e., aperture 1) of each target, and other apertures are number coded the same as in Figure 1. The gray contours marked by ”SF” represent the PAH band ratio distribution of 185 spatially resolved spaxels in 29 SF galaxies measured by L. Zhang et al. (2022). The open small circles indicate the PAH band ratios of 83 Seyferts with the measurements of all the three PAH features available from the literature (i.e., J. D. T. Smith et al. 2007; M. J. O’Dowd et al. 2009; A. M. Diamond-Stanic & G. H. Rieke 2010; J. F. Gallimore et al. 2010; D. A. Sales et al. 2010; PAH measurements of upper limits are excluded). The filled small circles indicate the PAH band ratios of 11 low-redshift quasars with all the three PAH features detected by Y. Xie & L. C. Ho (2022). The upward and downward small triangles, respectively, indicate the PAH band ratios of 29 giant H II regions in 3 Local Group galaxies measured by V. Lebouteiller et al. (2011) and 14 BCDs measured by L. K. Hunt et al. (2010) and V. Lebouteiller et al. (2011). The gray grids represent model predictions of PAH band ratios by D. Rigopoulou et al. (2024), respectively, for neutral (top grid, marked by “+”) and 70% ionized (bottom grid, marked by “×”) PAHs of different sizes (i.e., with carbon number N _C = 20−400 from the right boundary to the left boundary of each grid) in the interstellar radiation field (ISRF; the top boundary of each grid) and the 10³ × ISRF (the bottom boundary of each grid).