Observations of NGC 6537. Shadowed regions mark the LSR velocity ranges adopted for each feature and the dashed vertical line indicates the systemic velocity. The CO J = 3–2, J = 4–3 and J = 6–5 transitions are taken from Edwards et al. (2014). They are shown here just as an illustration of the line profile evolution between the low- and intermediate-J CO lines, but they are used in our quantitative study.