Histogram of the optical depth of line-free channels, shown before (pink) and after (green) weighting with their overlap in purple. The black solid line shows the Gaussian fit to the weighted subbands. Unweighted, the fit to the distribution has a mean of μ_uw = −1.4 × 10⁻³ and a standard deviation of σ_uw = 7.2 × 10⁻³. Weighted, the best-fit Gaussian has properties of μ_w = −1.3 × 10⁻³ and σ_w = 7.3 × 10⁻³. These subbands of M 82 were flagged and processed following the criteria of M14.