Contours at which CR heating equals PE heating in the I_UV/ζ₋₁₆ − Z′ parameter space. The solid, dashed, and dotted curves are for T = 10⁴, 10³, and 10² K respectively, and the different colors correspond to different values of n/ζ₋₁₆, from n/ζ₋₁₆ = 10 (leftmost curves) to n/ζ₋₁₆ = 0.1 (rightmost curves; see legend).