Overview of the central HCO⁺J = 4−3 and C¹⁸O J = 3−2 spectra shown in black and red respectively. All have been recentred so that the source velocity is at zero. Sources that are considered blue-skewed (i.e. δν < −0.25) have their names in blue, while those considered red-skewed (i.e. δν > 0.25) have their names in red. The number in the upper-right corner of each panel indicates what factor the spectra have been multiplied by.