Cas A spectra stacked for Cα RRLs across a range of redshifts. The plot on the left shows the optical depth integrated at the central velocities (−7.7 km s⁻¹ to +19.8 km s⁻¹, which encompasses the −47 and −38 km s⁻¹ components) at each redshift. The outlying peak at the redshift (z = −0.000158) of the −47 km s⁻¹ component is clearly visible. The histogram on the right shows the binned distribution of the integrated optical depth. Cas A α-transition stacking demonstrates our method in a regime with a high signal-to-noise ratio.