Column density ratios of [OH⁺]/[CH⁺] (top) and [CH⁺]/[ArH⁺] (bottom) toward the NE (v< −100 km s^-1) and SW (v> −100 km s^-1) lines of sight. The ratios are shown in black curves. The green and red curves are optical depths and translate into column densities with factors 2.85 × 10¹², 20.0 × 10¹², and 1.70 × 10¹² cm^-2 km^-1 s for CH⁺, OH⁺, and ArH⁺, respectively. Channels with S/N< 3 or opacity larger than 2 were flagged prior to calculate the ratios.