Emission line for the different tracers of the source sample towards the dust emission peak. The 88 633 MHz component of HCN(1−0) is shown together with the 86 340 MHz component of H¹³CN(1−0), HN¹³C(1−0), the 93 176 MHz isolated component of N₂H ⁺ (1−0), and C¹⁸O(2−1). Spectra have been multiplied by a scaling factor to allow the simultaneous comparison. Red dotted lines show the systemic LSR velocity of the sources evaluated from the well-determined frequencies of N₂H ⁺ (1−0), see Pagani et al. (2009).