Density of the points map of AME_{30 GHz}/N_HI vs. I_{12 μm}/R. We overplot the contours enclosing 50% of the pixels (internal contour) and 75% of the pixels (external contour). The r_P, r_S, and τ_K give the corresponding Pearson correlation coefficient, the Spearman rank-order correlation coefficient, and the Kendall’s tau, respectively. The same models as in Fig. 5 are shown with black and pink circles for spinning nano-carbon dust, m = 0.38 and 0.5 D, respectively, and green circles for spinning nano-silicate dust (case 2), each symbol showing a different value of f_CNM (see Sects. 5.2 and 5.3 for details). For spinning nano-carbon dust, we show models with the same f_CNM but with variations in the size distribution and dust abundance. The standard THEMIS size distribution with a_min = 0.4 nm and α = –5, where α is the a-C nano-particle power-law exponent, is shown for m = 0.38 D (black circles). We show this same model, but with an abundance increase of 10% (hexagons). The squares show a_min = 0.4 nm, α = –5.5, and an abundance decrease by 20%. The triangles show a_min = 0.43 nm and α = –5.5, and the diamonds show α_min = 0.37 nm and α = –4.75 with the standard model abundances.