Maps of external emission measure EM_ISM made from the measured optical depth τ_40,ISM (right-hand side map in Figure 5) combined with Equation (2), assuming Z = 1. We plot the results for three temperatures, 10, 100, and 10,000 K, relevant for our discussions of molecular clouds, the diffuse, infrared-emitting medium around Tycho, and the ISM warm ionized gas, respectively. The units of EM_ISM are pc cm⁻⁶.