The leftmost two panels ratios illustrate the similarity of the HCO⁺ and HCN ladders. The rightmost panels show that the HCO⁺/HCN intensity ratios remain constant over the full range of J = 1 − 0 CO luminosities. The J = 4 − 3 data shown include data from APEX and ALMA in addition to the IRAM data from this paper. Straight lines mark ratios of unity. Fit parameters are listed in Table 8.