Optical depth as a function of velocity for the RRL spectrum of M 82 stacked at z_M 82 = 0.00073 and smoothed with a Savitzky-Golay filter (top). In blue we show the spectrum we reproduced following the criteria of M14; the magenta line shows the best-fit Gaussian. The standard deviation in the line-free channels is 6.5 × 10⁻⁴. In gray we show the final M14 spectrum (courtesy L. Morabito). The bottom plot shows the coverage, or number of subband data points, within velocity bins of 36.7 km s⁻¹. The coverage count has been normalized by 23, which is the number of data points in the bin where the line peak falls.