CR heating rates of the protogalaxy ISM by (1) direct Coulomb heating, in black, up to 0.6 kpc, and (2) indirect X-ray heating, in red. The solid lines are the results calculated for ℛ_SN = 1 yr⁻¹, dotted lines for ℛ_SN = 0.1 yr⁻¹ and dot-dashed for ℛ_SN = 0.01 yr⁻¹. ℛ_SN determines the energy budget of the system, and so the CR heating power in both channels scales directly. The shaded regions show the stellar radiation “floor” for the IX process: any evolution that brings the IX line into this area will instead be dominated by the up-scattering of stellar-radiation by CR electrons, preventing the X-ray heating power from falling into this region. The lower region accounts for a stellar radiation field when ℛ_SN = 0.01 yr⁻¹, the intermediate case above, for ℛ_SN = 0.1 yr⁻¹, and the highest region arises for ℛ_SN = 1.0 yr⁻¹. We note that the DC heating rate falls to negligible levels outside the protogalaxy (not shown) due to the magnetic containment of the CRs.