Electron pressure as a function of electron temperature of the −47 km s⁻¹ velocity component toward Cas A. The red solid line shows the upper limit of a pressure-broadened line width. Shaded regions represent 1σ and 3σ uncertainty. The green shaded regions show the line-width constraints from the combined Doppler, pressure, and radiation broadening terms for T_r, 100 = 1400 K (S17). The orange shaded regions show the physical properties of the HRRLs constrained from modeling the integrated optical depth. The yellow shaded regions show the physical properties of the CRRLs constrained from modeling the integrated optical depth (O17). This plot shows the distinction between the HRRLs and CRRLs, where HRRLs arise from slightly colder, denser regions in the cloud.