Left: same as in Fig. 8, but applied to Cα transitions in Cas A spectra. Fits to the −47 km s−1 and −38 km s−1 components (and all higher order transitions) have been subtracted in the spectrum of Cas A. Residuals remaining from imperfect subtraction of the brightest α transitions from −47 km s−1 and −38 km s−1 components can be seen in (C). These results demonstrate the behavior of the method in the presence of narrow spectral lines with a low signal-to-noise ratio. The redshifts (zH = 0.000337 and zC = −0.000002, with respect to stacking for Cα) of the two detections are shown in red. Right: stacked spectrum of Hα (top) associated with the −47 km s−1 component, one of the lowest frequency detections of hydrogen RRLs. Cα (bottom) associated with the Orion Arm, detected at this frequency for the first time.
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