Comparison between the line shapes of the HCO⁺J = 3 → 2 (green histograms) and J = 4 → 3 (blue histograms) transitions toward a) the western and b) eastern nucleus of Arp 220 (from Sakamoto et al. 2009), with selected lines detected with Herschel/PACS (c–i). The dashed vertical line indicates our adopted systemic velocity (Sect. 2.2). The velocity scale for the OH 79 (163) μm doublet is relative to the red (blue) Λ-component, and the systemic velocity for the other Λ-component is also indicated. The velocity of the O 3₀₃ → 2₁₂ line in c) is also indicated.