Cross-correlation analysis of CH₄, NH₃, and ¹³CO. For each species, the upper panel shows the CCF (solid) and ACF (dashed) with the S/N indicated on the right-hand side. The lower panels present the residuals of CCF-ACF which are used to estimate the noise outside of the expected peak. The cross-correlation was performed on the observed spectrum centred in its rest frame, which resulted in peaks around 0 km s⁻¹.