Relative abundances of HCO⁺, HNC, and C₂H observed in absorption with respect to H₂ plotted against the local spaced-averaged mean H₂ density derived from CH (at left) and galactocentric radius (at right). Column and number densities of H₂ were derived from the CH absorption profile as discussed in Sect. 4 of the text. The mean relative abundances ± 1σ and power-law slopes of the fitted regression lines are shown in the individual panels.