(Left) Radial profiles of |$I_{\rm {PAH}}/I_{\rm {TIR}}$| averaged over the pixels within a circular region of a radius |$r$|⁠. (Right) Same as the left-hand panel, but all the profiles are normalized. The profiles are color-coded with the Galactocentric distance; the violet, blue, green, orange, and red curves correspond to 0–3, 3–5, 5–7, 7–9, and 9–|$12\:$|kpc from the Galactic center, respectively, while the bold curves are their averages. (Color online)